Add os-assisted-volume-snapshots extension

Add a new API extension that exposes assisted volume snapshot
capabilities.  This extension is admin only by default.  We expect it to
only be called by Cinder.  If you have your deployment set up in such a
way that your adminURL is different from the public, this extension can
only be loaded in the admin API instance.  Cinder will pull that URL out
of the service catalog to use.

Part of blueprint qemu-assisted-snapshots

Change-Id: I79e22ab6ef66fa16dc534a4336e766065702b2f5
Merge "Add os-assisted-volume-snapshots extension"