Add os-server-external-events support

This adds support for the os-server-external-events extension in nova,
which allows other services to deliver events to nova.

It also adds a shell command to trigger the "network-changed" event
manually, which will cause nova to refresh its network cache from
neutron.

Related to blueprint admin-event-callback-api

Change-Id: I1a302a43b6b7a6d8bdc03965a8f4c1a151bcab88
Merge "Add os-server-external-events support"