Fix 'NoneType' object has no attribute 'get'

There are situations where the action object returned by libjuju may not
have the 'results' attribute which leads to using the default value when
running ".get('results')" which it's None.

This change sets the default value to an empty dictionary to allow the
following ".get('Stdout', '')" succeed.

zaza/openstack/utilities/openstack.py

@@ -238,7 +238,7 @@ async def _check_ca_present(model, ca_files):
             for unit in units:
                 try:
                     output = await unit.run('cat {}'.format(ca_file))
-                    contents = output.data.get('results').get('Stdout', '')
+                    contents = output.data.get('results', {}).get('Stdout', '')
                     if ca_cert not in contents:
                         break
                 # libjuju throws a generic error for connection failure. So we