Expose the storage backend being used
Expose the storage backend being used

To match how the persistence connections
provide access to there backend it seems
appropriate to also have the storage layer
provide the same property so that users of
it can easily access the underlying backend
object (if they so need to).

Change-Id: I8fdb5e7efc66bba93da2949bb6adad534bf3b127

openstack/taskflow,pombredanne/taskflow-1,openstack/taskflow,junneyang/taskflow,junneyang/taskflow,jimbobhickville/taskflow,jimbobhickville/taskflow,pombredanne/taskflow-1