Instance should only become 'ACTIVE' after migration completes

OS-268

An instance's task_state should only switch from 'migrating' to None
after migration completes.

Change-Id: I503953806afe242e6fb73514080c731c9f6ed2d3
Merge remote-tracking branch 'origin/grizzly/1.4' into grizzly15

Conflicts fixed:
	cobalt/nova/extension/manager.py

Change-Id: Ifb3f6ab6e653c15a9cf5aeeaa7d8ad3e6fb19253