Default the impl_memory conf to none

This configuration isn't used for anything in the memory
backend so default it to none so that it doesn't need to
be provided (since it's not used).

Change-Id: I9e333aec52b697ebd3d5b1f4ec14d810e99bab19

taskflow/persistence/backends/impl_memory.py

@@ -32,7 +32,7 @@ class MemoryBackend(base.Backend):
     """A backend that writes logbooks, flow details, and task details to in
     memory dictionaries.
     """
-    def __init__(self, conf):
+    def __init__(self, conf=None):
         super(MemoryBackend, self).__init__(conf)
         self._log_books = {}
         self._flow_details = {}