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

taskflow/storage.py

@@ -271,6 +271,11 @@ class Storage(object):
         # This never changes (so no read locking needed).
         return self._flowdetail.uuid
 
+    @property
+    def backend(self):
+        # This never changes (so no read locking needed).
+        return self._backend
+
     def _save_flow_detail(self, conn):
         # NOTE(harlowja): we need to update our contained flow detail if
         # the result of the update actually added more (aka another process