Evacuate each instance from one host to another

Added a new extension that adds the ability for admins to evacuate an
entire host to another host. This internally uses the
existing server.evacuate api.
The target host is optional so that a free host will be chosen by the
scheduler in the api.
Implements: blueprint evacuate-host

Change-Id: I2352836d01952e281e15edb9bdd1b912106516d6
Merge "Evacuate each instance from one host to another"