Updating authentication to use keystone sessions

Sahara currently uses per access authentication when creating OpenStack clients. This style of authentication requires keystone connections on every client creation. The keystone project has created a mechanism to streamline and improve this process in the form of Session objects. These objects encapsulate mechanisms for updating authentication tokens, caching of connections, and a single point for security improvements. Sahara should migrate its OpenStack client objects to use session objects for all clients.