Add test case for volume unmanage and manage

Lots of Cinder backend drivers have support unmanage and manage
features. It's necessary to add test case for these feature apis.

Including:

[1] Add volume manage client as library
[2] Add unmanage volume api to v2 volumes client
[3] Add release note
[4] Add test case: test_volume_unmanage_manage

Depends-On: I055aa66738deb5ae2fb925429cec565e3901340c
Change-Id: I4294de92a801db2697ba870ff1b5fdbe4b2f5a4b
8 files changed