Add support for mongodb 2.4 roles and otherDBRoles and implicit auth

Hi, this add support for mongodb 2.4's fine grained security roles.

It also adds an optional "check_implicit_auth" parameter: When you enable admin on mongodb, but haven't created any admin users yet, it starts up, and allows logins with no username/password. Enabling this option means ansible will first try and connect with no username/password even if you supply one. Solves an irritating bootstrap problem after enabling auth for the first time.