Set default local_dc in distance in DCAwareRoundRobinPolicy

If `distance` is called before `on_add/up` then we would end up
with null `local_dc` value if the value wasn't specified in the
constructor.

Author: sylwiaszunejko

cassandra/policies.py

@@ -264,6 +264,9 @@ def populate(self, cluster, hosts):
 
     def distance(self, host):
         dc = self._dc(host)
+        if not self.local_dc:
+            self.local_dc = dc
+            return HostDistance.LOCAL
         if dc == self.local_dc:
             return HostDistance.LOCAL