Fix the default prototype for join_group

Otherwise it's different than the one used in the ZooKeeper
implementation.

Change-Id: I1ba3af04756a6b27a6d09c8c7b76a213ec9bf359

tooz/coordination.py

@@ -63,7 +63,7 @@ class CoordinationDriver(object):
         """
 
     @abc.abstractmethod
-    def join_group(self, group_id, capabilities):
+    def join_group(self, group_id, capabilities=b""):
         """Join a group and establish group membership asynchronously.
 
         :param group_id: the id of the group to join