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
Use Python 3.x compatible except: construct

Per (proposed) hacking H203 check, use the non-deprecated
except x as y: construct, which works with any
Python version >= 2.6

Change-Id: Ib7cab00cb8f219154663a4d51a855a1a2718e8cb
Merge "Evacuate each instance from one host to another"