Add Host Maintenance Mode Get and evacuation support

To support GET the "host maintenance mode" from PowerVC by
PowerVC Driver, and evacuation all host out of the maintenance
entered host by PowerVC Driver. The design finally goes to Implement a
new nova rest-API for Get/Set maintenance status, and evacuate
vm during maintenance.

Change-Id: I9fba6b47e143ae8ea2165573302448fcdc61406f
Closes-Bug: 1372730
Merge "Add Host Maintenance Mode Get and evacuation support"