Use None as get_nova_session_client() version default

This allows callers to pass None and let get_nova_session_client() to
use a sane default API, specifically this allows intermediate users
(e.g. launch_guest() ) to proxy values passed by the caller.

zaza/openstack/utilities/openstack.py

@@ -367,7 +367,7 @@ def get_designate_session_client(**kwargs):
                            **kwargs)
 
 
-def get_nova_session_client(session, version=2):
+def get_nova_session_client(session, version=None):
     """Return novaclient authenticated by keystone session.
 
     :param session: Keystone session object
@@ -377,6 +377,8 @@ def get_nova_session_client(session, version=2):
     :returns: Authenticated novaclient
     :rtype: novaclient.Client object
     """
+    if not version:
+        version = 2
     return novaclient_client.Client(version, session=session)