Default registry to docker hub

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

@@ -33,8 +33,9 @@ description:
 options:
   registry:
     description:
-       - URL of the registry, for example: https://index.docker.io/v1/
+       - URL of the registry, defaults to: https://index.docker.io/v1/
-    required: true
+    required: false
+    default: https://index.docker.io/v1/
   username:
     description:
        - The username for the registry account
@@ -214,7 +215,7 @@ def main():
 
     module = AnsibleModule(
         argument_spec = dict(
-            registry        = dict(required=True),
+            registry        = dict(required=False, default='https://index.docker.io/v1/'),
             username        = dict(required=True),
             password        = dict(required=True),
             email           = dict(required=False, default='anonymous@localhost.local'),