Merge pull request #502 from jeffbrl/cloud-init

Add initial cloud-init documentation

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+.. _cloud-init:
+
+###############
+VyOS cloud-init
+###############
+
+Cloud instances of VyOS are initialized using the industry-standard cloud-init. 
+Via cloud-init, the system performs tasks such as injecting SSH keys and
+configuring the network. In addition, the user can supply a custom
+configuration at the time of instance launch.
+
+**************
+Config Sources
+**************
+
+VyOS support three type of config sources.
+
+.. stop_vyoslinter
+
+* Metadata - Metadata is sourced by the cloud platform or hypervisor. In some clouds, there is implemented as an HTTP endpoint at http://169.254.169.254.
+
+* Network configuration - Ths config source informs the system about the network.
+
+* User-data - User-data is specified by the user. This config source offers the most flexibility and will be the focus of this documentation.
+
+.. start_vyoslinter
+
+
+*********
+User-data
+*********
+
+Major cloud providers offer a means of providing user-data at the time
+of instance launch. Typically the user includes user-data as plain
+text and the cloud provider's platform base64 encodes the user-data
+before injecting it into the instance. 
+
+VyOS implements a user-data format called cloud-config.
+
+
+************************
+cloud-config file format
+************************
+
+
+A cloud-config document is written in YAML. The file must begin
+with "#cloud-config". The key used to designate a VyOS configuration
+is "vyos_config_commands". What follows is VyOS configuration using
+the "set-style" syntax. Both "set" and "delete" commands are supported.
+
+Commands requirements:
+
+* one command per line
+* if command ends in a value, it must be inside single quotes
+* a single-quote symbol is not allowed inside command or value
+
+
+The commands list produced by the `show configuration commands` command on a
+VyOS router should comply with all the requirements, so it is easy to get a 
+proper commands list by copying it from another router.
+
+The configuration specified in the cloud-config document is merged with
+the default configuration and saved to /config/config.boot.
+
+Here is an example cloud-config.
+
+.. code-block:: yaml
+
+   #cloud-config
+   vyos_config_commands:
+     - set system host-name 'vyos-prod-ashburn'
+     - set system ntp server 1.pool.ntp.org
+     - set system ntp server 2.pool.ntp.org
+     - delete interfaces ethernet eth1 address 'dhcp'
+     - set interfaces ethernet eth1 address '172.31.7.247/20'
+     - set protocols static route '172.31.0.0/16' next-hop '100.64.16.1'
+
+*************************
+System Defaults/Fallbacks
+*************************
+
+These are the VyOS defaults and fallbacks.
+
+* SSH is configured on port 22
+* vyos/vyos credentials if no others specified by data source
+* DHCP on first Ethernet interface if no network configuration is provided
+
+
+All of these can be overridden using configuration in user-data.
+
+
+***************
+Troubleshooting
+***************
+
+If you encounter problems, verify that the cloud-config document contains
+valid YAML. Online resources such as https://yamlvalidator.com/ provide
+a simple tool for validating YAML.
+
+cloud-init logs to /var/log/cloud-init.log. This file can be helpful in
+determining why the configuration varies from what you expect.
+