No need for .keys on volumes list

Since c3f92cca21 changed "volumes" to be a list instead of a dictionary, we don't need (and cannot) .keys when appending to set.
Reported as bug #1957

lib/ansible/modules/cloud/docker/docker.py

@@ -1025,7 +1025,7 @@ class DockerManager(object):
 
             expected_volume_keys = set((image['ContainerConfig']['Volumes'] or {}).keys())
             if self.volumes:
-                expected_volume_keys.update(self.volumes.keys())
+                expected_volume_keys.update(self.volumes)
 
             actual_volume_keys = set((container['Config']['Volumes'] or {}).keys())