Add support for running python-manta internally, on Manta itself

Sometimes it's useful to access Manta in a compute step, e.g. when you want to customize output and `mpipe` isn't cutting it. When in a compute step, you're already implicitly authenticated and so there's no need to sign the request – so the client shouldn't complain when you don't provide a signature or signer.

Tested and works for me.

(Pony: also, it would be nice to have this package installed by default on compute instances. :-))