Rebuilding stopped instance should not set terminated_at

At the end of a rebuild action, nova checks the original vm_state
of the instance to see if it was stopped. If the instance was
originality stopped, nova will stop the newly rebuild instance.
Right before the stop, nova updates the instance and sets it's
terminated_at to utcnow(). Due to this, the periodic usage audit
does not send usage for the instance.

There's no reason for setting tereminated_at here, it should only
be set as part of instance deletion

Fixes bug: 1189554

Change-Id: I141aba1b4f3c15a4c17e1fc401ae9479acafb23f
Merge "Rebuilding stopped instance should not set terminated_at"