First implementation of cloning from the replica.

At the moment we just replace the master with the
node at the 'clonefrom' tag if it's present. Master
should be available anyway, otherwise, it will not
even try to do cloning.

Acceptance tests:
https://github.com/zalando/patroni/pull/144/commits

tests/test_ha.py

@@ -57,6 +57,7 @@ class MockPatroni(object):
         self.nap_time = 10
         self.replicatefrom = None
         self.api.connection_string = 'http://127.0.0.1:8008'
+        self.clonefrom = None
 
 
 def run_async(func, args=()):