Update default Nova API version to 2.53(Pike)

Services are now identified by uuid instead of database id to ensure
uniqueness across cells.
GET /os-services returns a uuid in the id field of the response
from API microversion 2.53(maximum in Pike)

This patch set updates default Nova API version to 2.53.

Change-Id: Ib9fefb794eda3c9e75c6a2f5cfdb0e682b8955f3
Closes-Bug: #1709544

watcher/conf/nova_client.py

@@ -23,7 +23,7 @@ nova_client = cfg.OptGroup(name='nova_client',
 
 NOVA_CLIENT_OPTS = [
     cfg.StrOpt('api_version',
-               default='2',
+               default='2.53',
                help='Version of Nova API to use in novaclient.'),
     cfg.StrOpt('endpoint_type',
                default='publicURL',