This was spotted from someone validating the fix for
bug 1670627. They reported that even though they failed
to delete an instance in ERROR state that was in cell0,
the quota usage was decremented.
This is because we committed the quota reservation
to decrement the usage before actually attempting to destroy
the instance, rather than upon successful deletion.
The rollback after InstanceNotFound is a noop because of
how the Quotas.rollback method noops if the reservations
were already committed. That is in itself arguably a bug,
but not fixed here, especially since the counting quotas
work in Pike will remove all of the reservations commit and
rollback code.
Change-Id: I12d1fa1a10f9014863123ac9cc3c63ddfb48378e
Partial-Bug: #1678326
032937ce51