Allow specification of erlang node name for rabbitmq_user/vhost/parameter actions.

In our environment, we need to run multiple rabbits on a single box, so we need to be able to specify the Erlang node name when configuring users, parameters, and vhosts.

I've not been able to verify 100% that the parameters changes are correct, unfortunately, as we don't have anything that accepts them; but they're isomorphic to the other changes, and the invocation of `rabbitmqctl` looks correct to my eyes.