Add support to set maintance mode for PowerVC

[Jerry Cai]

As a cloud admin, I want to set the host managed by PowerVC to "maintenance mode" and disable the "maintenance mode" support, thus I can do some maintenance work on that host.

For detail, please refer to workitem: https://jazz04.rchland.ibm.com:9443/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/184941

[Jerry Cai (caimin)]

review url: https://review.openstack.org/#/c/118235/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/118235
Committed: https://git.openstack.org/cgit/stackforge/powervc-driver/commit/?id=1c7474486fe7d5a488cd98f4de712a94be5059bf
Submitter: Jenkins
Branch: master

commit 1c7474486fe7d5a488cd98f4de712a94be5059bf
Author: Jerry Cai <email address hidden>
Date: Mon Sep 1 17:34:54 2014 +0800

    Add host maintenance mode support for PowerVC Driver

    As a cloud admin, it is able to set the host managed by PowerVC to
    "maintenance mode" and disable the "maintenance mode" support, through
    PowerVC Driver.
    1. If set maintenance mode to "true" it responses: "on_maintenance"
    2. If set maintenance mode to "false" it responses: "off_maintenance"
    3. If set maintenance mode to "true" when the host is already in
    maintenance mode, it will report the error message from PowerVC to user.

    Change-Id: I59ef830244202a4337ad72292e6341406b8464d3
    Closes-Bug: #1363912