Set the Pike release version for scheduler RPC

Since later changes in this series is assuming that the scheduler will
always receive instance_uuids in the select_destinations() call, we need
to clearly demarcate that Pike uses version 4.4 for the scheduler RPC.

Blueprint: return-alternate-hosts

Change-Id: Ibee7721cc3a9856f9c2bc252e68eb8b67e33ea3d

nova/scheduler/rpcapi.py

@@ -109,6 +109,7 @@ class SchedulerAPI(object):
         'mitaka': '4.3',
         'newton': '4.3',
         'ocata': '4.3',
+        'pike': '4.4',
     }
 
     def __init__(self):