Fix wrong quota reservation when deleting resizing instance

When we delete an instance which is resizing from m1.small to m1.tiny, we use
instance['memory_mb'] to create quota's ram reservation, if the resizing is not
'finished', the in_use value in quota_usages is NOT commited(2048M),
but the instance['memory_mb'] is updated(512M) in _finish_resize(), so the
deleting operation after updating will commit a wrong ram reservation(-512M),
and the in_use value changes to 2048-512=1536.
The cores reservation encounters the same problem.

Fixes bug #1099729

Change-Id: Ide54fc9d822041f535819c496822aeac5a8dc68d
Merge "Fix wrong quota reservation when deleting resizing instance"