Fix Code becoming integer.

Value of the Code key in the returned Dict is str in master branch. However
while trying to get compatible with juju 3.x series it became int. Now it is
not conforming to the function signature. Also there are usages in some
tests so it reverted back to str.

zaza/model.py

@@ -546,7 +546,7 @@ def _normalise_action_results(results):
             elif results.get(old_key) and not results.get(key):
                 results[key] = results[old_key]
         if 'return-code' in results:
-            results['Code'] = results.get('return-code')
+            results['Code'] = str(results.get('return-code'))
             del results['return-code']
         return results
     else: