Improve user creation

[alsyia]

Hi,

Thanks for this role! I've been using it for a few days and I noticed a few things about how RabbitMQ users are managed:

• Creation/deletion tasks are run on every node, this can lead to a race condition if nodes are clustered: all nodes thinks they should add/remove the user but only the first one gets to do it, others raise an error
• Also, the password is logged by Ansible during the creation like this:

ok: [my-rabbit-server] => (item={'password': 'my-password', 'tags': 'administrator', 'user': 'my-user'})

This PR contains fixes for these two points, do you think it could be merged?

Thank you!

P.S. : Probably more PRs to come :)

[jpiron]

Hi any feedback on this ?