azure: support bouncing interfaces to publish new hostname

See the added doc/sources/azure/README.rst for why this is necessary.
Essentially, we now are doing the following in the get_data() method
of azure datasource to publish this NewHostname:
 hostname NewHostName
 ifdown eth0; 
 ifup eth0

LP: #1202758

1 files changed, 134 insertions, 0 deletions

diff --git a/doc/sources/azure/README.rst b/doc/sources/azure/README.rst
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+================
+Azure Datasource
+================
+
+This datasource finds metadata and user-data from the Azure cloud platform.
+
+Azure Platform
+--------------
+The azure cloud-platform provides initial data to an instance via an attached
+CD formated in UDF.  That CD contains a 'ovf-env.xml' file that provides some
+information.  Additional information is obtained via interaction with the
+"endpoint".  The ip address of the endpoint is advertised to the instance
+inside of dhcp option 245.  On ubuntu, that can be seen in
+/var/lib/dhcp/dhclient.eth0.leases as a colon delimited hex value (example:
+``option unknown-245 64:41:60:82;`` is 100.65.96.130)
+
+walinuxagent
+------------
+In order to operate correctly, cloud-init needs walinuxagent to provide much
+of the interaction with azure.  In addition to "provisioning" code, walinux
+does the following on the agent is a long running daemon that handles the
+following things:
+- generate a x509 certificate and send that to the endpoint
+
+waagent.conf config
+~~~~~~~~~~~~~~~~~~~
+in order to use waagent.conf with cloud-init, the following settings are recommended.  Other values can be changed or set to the defaults.
+
+  ::
+
+   # disabling provisioning turns off all 'Provisioning.*' function
+   Provisioning.Enabled=n
+   # this is currently not handled by cloud-init, so let walinuxagent do it.
+   ResourceDisk.Format=y
+   ResourceDisk.MountPoint=/mnt
+
+
+Userdata
+--------
+Userdata is provided to cloud-init inside the ovf-env.xml file. Cloud-init
+expects that user-data will be provided as base64 encoded value inside the
+text child of a element named ``UserData`` or ``CustomData`` which is a direct
+child of the ``LinuxProvisioningConfigurationSet`` (a sibling to ``UserName``)
+If both ``UserData`` and ``CustomData`` are provided behavior is undefined on
+which will be selected.
+
+In the example below, user-data provided is 'this is my userdata', and the
+datasource config provided is ``{"agent_command": ["start", "walinuxagent"]}``.
+That agent command will take affect as if it were specified in system config.
+
+Example:
+
+.. code::
+
+ <wa:ProvisioningSection>
+  <wa:Version>1.0</wa:Version>
+  <LinuxProvisioningConfigurationSet
+     xmlns="http://schemas.microsoft.com/windowsazure"
+     xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
+   <ConfigurationSetType>LinuxProvisioningConfiguration</ConfigurationSetType>
+   <HostName>myHost</HostName>
+   <UserName>myuser</UserName>
+   <UserPassword/>
+   <CustomData>dGhpcyBpcyBteSB1c2VyZGF0YQ===</CustomData>
+   <dscfg>eyJhZ2VudF9jb21tYW5kIjogWyJzdGFydCIsICJ3YWxpbnV4YWdlbnQiXX0=</dscfg>
+   <DisableSshPasswordAuthentication>true</DisableSshPasswordAuthentication>
+   <SSH>
+    <PublicKeys>
+     <PublicKey>
+      <Fingerprint>6BE7A7C3C8A8F4B123CCA5D0C2F1BE4CA7B63ED7</Fingerprint>
+      <Path>this-value-unused</Path>
+     </PublicKey>
+    </PublicKeys>
+   </SSH>
+   </LinuxProvisioningConfigurationSet>
+ </wa:ProvisioningSection>
+
+Configuration
+-------------
+Configuration for the datasource can be read from the system config's or set
+via the `dscfg` entry in the `LinuxProvisioningConfigurationSet`.  Content in
+dscfg node is expected to be base64 encoded yaml content, and it will be
+merged into the 'datasource: Azure' entry.
+
+The '``hostname_bounce: command``' entry can be either the literal string
+'builtin' or a command to execute.  The command will be invoked after the
+hostname is set, and will have the 'interface' in its environment.  If
+``set_hostname`` is not true, then ``hostname_bounce`` will be ignored.
+
+An example might be:
+  command:  ["sh", "-c", "killall dhclient; dhclient $interface"]
+
+.. code::
+
+  datasource:
+   agent_command
+   Azure:
+    agent_command: [service, walinuxagent, start]
+    set_hostname: True
+    hostname_bounce:
+     # the name of the interface to bounce
+     interface: eth0
+     # policy can be 'on', 'off' or 'force'
+     policy: on
+     # the method 'bounce' command.
+     command: "builtin"
+     hostname_command: "hostname"
+    }
+
+hostname
+--------
+When the user launches an instance, they provide a hostname for that instance.
+The hostname is provided to the instance in the ovf-env.xml file as
+``HostName``.
+
+Whatever value the instance provides in its dhcp request will resolve in the
+domain returned in the 'search' request.
+
+The interesting issue is that a generic image will already have a hostname
+configured.  The ubuntu cloud images have 'ubuntu' as the hostname of the
+system, and the initial dhcp request on eth0 is not guaranteed to occur after
+the datasource code has been run.  So, on first boot, that initial value will
+be sent in the dhcp request and *that* value will resolve.
+
+In order to make the ``HostName`` provided in the ovf-env.xml resolve, a
+dhcp request must be made with the new value.  Walinuxagent (in its current
+version) handles this by polling the state of hostname and bouncing ('``ifdown
+eth0; ifup eth0``' the network interface if it sees that a change has been
+made.
+
+cloud-init handles this by setting the hostname in the DataSource's 'get_data'
+method via '``hostname $HostName``', and then bouncing the interface.  This
+behavior can be configured or disabled in the datasource config.  See
+'Configuration' above.