Add support for discovering the available glance versions

[Flavio Percoco]

Glanceclient can't query the `/versions` endpoint right now and it makes it impossible for the client to discover the available versions on the server.

Example use case: https://review.openstack.org/#/c/222150/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/240183

[wangxiyuan (wangxiyuan)]

v2 use schema to wrap the return value. But there is no schema about version. So Glance should also add it if needed.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/240183
Committed: https://git.openstack.org/cgit/openstack/python-glanceclient/commit/?id=1f1934eb8277c7432e56da024bd8ce2171f3e8e6
Submitter: Jenkins
Branch: master

commit 1f1934eb8277c7432e56da024bd8ce2171f3e8e6
Author: wangxiyuan <email address hidden>
Date: Thu Oct 29 17:50:36 2015 +0800

    Add versions list function

    This patch add a function to query the Glance API versions.

    DocImpact

    Change-Id: I21eeaee3db4ae23f608b68bb319772ac612975b4
    Closes-bug: #1511180