Add 'rollback' command as allowed for failure_action

Signed-off-by: Nikolay Murga <work@murga.kiev.ua>

docker/types/services.py

@@ -371,7 +371,7 @@ class UpdateConfig(dict):
         delay (int): Amount of time between updates.
         failure_action (string): Action to take if an updated task fails to
           run, or stops running during the update. Acceptable values are
-          ``continue`` and ``pause``. Default: ``continue``
+          ``continue``, ``rollback`` and ``pause``. Default: ``continue``
         monitor (int): Amount of time to monitor each updated task for
           failures, in nanoseconds.
         max_failure_ratio (float): The fraction of tasks that may fail during
@@ -385,7 +385,7 @@ class UpdateConfig(dict):
         self['Parallelism'] = parallelism
         if delay is not None:
             self['Delay'] = delay
-        if failure_action not in ('pause', 'continue'):
+        if failure_action not in ('pause', 'continue', 'rollback'):
             raise errors.InvalidArgument(
                 'failure_action must be either `pause` or `continue`.'
             )