Elevate context for delete volume with no host.

So in the case of a volume that is placed in error state
on create and never actually deployed, the cinder.volume.api delete
call does a short cut call to db.destroy_volume which is fine because
all we have is a DB entry (scheduler never deployed the volume).

Unfortunately, this requires admin context, so just add an elevate
context to the db.destroy_volume call.

Fixes bug: 1084273

Change-Id: I0ef8bf4356047c385bef703b8dce7d5edf537bf6
(cherry picked from commit a2830e5417539980249e3ab12d8bf9538d8956c4)