* Allow creation of user with localhost exception.
Fixes#22791
When access control is enabled, Mongo allows a user to be created from
localhost (called the "localhost exception":
https://docs.mongodb.com/v3.2/core/security-users/#localhost-exception).
When the `update_password` parameter was added to this module in
Ansible 2.1, this functionality was broken due to a query performed
before `user_add()` is called. This fix only performs the query when
when `update_password` is set to `on-create`, allowing a user to be
created via the localhost exception.
* Only set `password = None` when user exists.
44730c28cc