Add nova CLI extension for host maintenance mode status query/evaucation

Add a nova cli "nova host-maintenance" to query and enable/disable
the host maintenance mode for a specified host.

For detail usage, please refer to: "nova help host-maintenance".

Change-Id: Ibc3f92521546a82672b1619f7b65416909d92dfc
Closes-Bug: 1386026
(cherry picked from commit 40e90593e3ec084509a8fb1df346ecab894a6514)