expose quiesce unquiesce api

Currently the Nova provide VM snapshot API, which will take a consistency snapshot of a VM and regarding volumes, and will quiesce/unquiesce VM automatily with guest agent support.

In NFV scenario, a VNF (telecom application) often is consisted of a group of VMs. To make it be able to restore in another site for catastrophic failures happened, this group of VMs snapshot/backup/restore should be done in a transaction way to guarantee the application level consistency but not only on single VM level : for example, quiesce VM1, quiesce VM2, quiesce VM3, snapshot VM1's volumes, snapshot VM2's volumes, snapshot VM3's volumes, unquiesce VM3, unquiesce VM2, unquiesce VM1. For some telecom application, the order is very important for a group of VMs with strong relationship.

Provide atomic quiesce / unquiesce API from Nova, to make consistency snapshot of a group of VMs in a transaction way is possible.

Refer to https://gerrit.opnfv.org/gerrit/#/c/1438/3/multisite-vnf-gr-requirement.rst for more detail description.
Minutes of OPNFV multisite project meeting: http://ircbot.wl.linuxfoundation.org/meetings/opnfv-meeting/2015/opnfv-meeting.2015-09-03-08.02.html