Ability to execute database commands on an ARBITER

Admittedly, there's not a ton of interesting stuff to do on an arbiter.
But there are a few commands you might want to run there, like replSetGetStatus or shutdown.

Right now, pymongo forbids all connections to arbiters (cf. https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/connection.py#L538 & https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/connection.py#L558 )

If it would permit a direct connection to an arbiter (i.e., a connection specifying just that node), then these commands could be issued via such a connection. That would be handy.