Update the default value for nova api_verison
Update the default value for nova api_verison

refer to https://github.com/openstack/watcher/blob/master/watcher/conf/nova_client.py#L26

Change-Id: If7c12d49c68e1bfc30327d465b9d5bafe82882e0

stackforge/watcher,stackforge/watcher,openstack/watcher,openstack/watcher