Allow deleting instances while uuid lock is held

Avoid issues where locked up operations on the instance prevent it
from being deleted by the user. Reasons for the lock up are a separate
issue and will be handled elsewhere, but terminating the instance
should not affect any tasks anyway. Any modification should already
gracefully handle the instance going away.

For more discussion about the issue see:
http://lists.openstack.org/pipermail/openstack-dev/2013-October/017454.html

Partial-Bug: #1248563

Change-Id: Ife26ffefccea11d6e9ca5fbd970192b322fc04fd
Merge "Allow deleting instances while uuid lock is held"