Default net to 'bridge' in container diff

This prevents an unnecessary reload when the `net` parameter is unspecified
(i.e. almost always).

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

@@ -1115,8 +1115,8 @@ class DockerManager(object):
 
             # NETWORK MODE
 
-            expected_netmode = self.module.params.get('net') or ''
-            actual_netmode = container['HostConfig']['NetworkMode']
+            expected_netmode = self.module.params.get('net') or 'bridge'
+            actual_netmode = container['HostConfig']['NetworkMode'] or 'bridge'
             if actual_netmode != expected_netmode:
                 self.reload_reasons.append('net ({0} => {1})'.format(actual_netmode, expected_netmode))
                 differing.append(container)