Allow user to pass 'ex_auth_instance' OpenStackBaseConnection class constructor.

This way a single OpenStackAuthConnection instance can be more easily re-used
across multiple driver instances.


git-svn-id: https://svn.apache.org/repos/asf/libcloud/branches/0.12.x@1485904 13f79535-47bb-0310-9956-ffa450edef68