allows a mgmt user to migrate an instance to a specific host

a mgmt user and use the instance migrate api and specify a host that
they would like the instance to move to.
Fixing migration fake mode.

implements blueprint migration-force-to-host
https://blueprints.launchpad.net/reddwarf/+spec/migration-force-to-host

Change-Id: I1fd7b2f5ae5328de642ec1ad6ee4d62c5a29168f
Merge "allows a mgmt user to migrate an instance to a specific host"