Initial hotplug support (#936)

Adds a udev script which will invoke a hotplug hook script on all net
add events. The script will write some udev arguments to a systemd FIFO
socket (to ensure we have only instance of cloud-init running at a
time), which is then read by a new service that calls a new 'cloud-init
devel hotplug-hook' command to handle the new event.

This hotplug-hook command will:
- Fetch the pickled datsource
- Verify that the hotplug event is supported/enabled
- Update the metadata for the datasource
- Ensure the hotplugged device exists within the datasource
- Apply the config change on the datasource metadata
- Bring up the new interface (or apply global network configuration)
- Save the updated metadata back to the pickle cache

Also scattered in some unrelated typing where helpful

1 files changed, 4 insertions, 0 deletions

diff --git a/doc/rtd/topics/cli.rst b/doc/rtd/topics/cli.rst
index 0ff230b5..b6115ed6 100644
--- a/doc/rtd/topics/cli.rst
+++ b/doc/rtd/topics/cli.rst
@@ -119,6 +119,10 @@ Current subcommands:
    schema errors locally without the need for deployment. Schema
    validation is work in progress and supports a subset of cloud-config
    modules.
+ * ``hotplug-hook``: respond to newly added system devices by retrieving
+   updated system metadata and bringing up/down the corresponding device.
+   This command is intended to be called via a systemd service and is
+   not considered user-accessible except for debugging purposes.
 
 
 .. _cli_features: