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, 2 insertions, 2 deletions

diff --git a/cloudinit/stages.py b/cloudinit/stages.py
index 06e0d9b1..bc164fa0 100644
--- a/cloudinit/stages.py
+++ b/cloudinit/stages.py
@@ -241,7 +241,7 @@ class Init(object):
             else:
                 return (None, "cache invalid in datasource: %s" % ds)
 
-    def _get_data_source(self, existing):
+    def _get_data_source(self, existing) -> sources.DataSource:
         if self.datasource is not NULL_DATA_SOURCE:
             return self.datasource
 
@@ -267,7 +267,7 @@ class Init(object):
                                                cfg_list,
                                                pkg_list, self.reporter)
             LOG.info("Loaded datasource %s - %s", dsname, ds)
-        self.datasource = ds
+        self.datasource = ds  # type: sources.DataSource
         # Ensure we adjust our path members datasource
         # now that we have one (thus allowing ipath to be used)
         self._reset()