Cleanup 'deleting' instances on restart

In case some instance was marked as deleting but for some reason didn't
finish (for example request was stuck in libvirt), retry the delete at
startup. This happens at startup and on the host owning the instance,
so there's no reason to use the lock.

Change-Id: Iad18e9a7c6cb8e272e67a82284127ad895441dcf
Partial-Bug: #1248563
Closes-Bug: 1247174
Implements: blueprint recover-stuck-state
Merge "Cleanup 'deleting' instances on restart"