Update Instance from database after destroy

In cases where Nova falls back to simply locally deleting an
instance, the Instance object was not updated with the deleted_at
generated by the database implementation. Due to this, the
notification emitted to signify that the delete finished was
getting emitted without a deleted_at value.

This change uses the result from the instance_destroy call to
update the Instance object with the latest state from the database.
Thus, the compute.instance.delete.end notification is emitted
with the proper deleted_at value.

Closes-Bug: #1250206
Change-Id: I55ed37cf89dd6eb8ea901c6f813191a0ce1058e8
Merge "Update Instance from database after destroy"