New MongoDB role

New simple MongoDB role
Ansible has {{ ansible_user_id }} but that's default vagrant unless you use sudo, then it's root.
And we could add a global var {{ phansible_root_user }} , but I don't see the advantage of making the root user configurable. Because normally you don't want that other users then root can edit the config file, right?

And what do you mean with running the playbook with a defined user from the command line?