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 commit 'Become 'ACTIVE' AFTER migration completes (OS-268)' into havana/1.4

Conflicts:
	cobalt/nova/extension/manager.py

Change-Id: I5da1121eaaf7aaa2c1a1c615b0d1a5eef95ec772