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

[debrouwere]

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. :-))

[trentm]

@stdbrouw So sorry for it being months with no comment on this. FWIW there was some work in https://github.com/joyent/python-manta/blob/master/CHANGES.md#210 (for #18) which I think solves this... if not the Pony bit. I'll open an (internal) issue to see about getting python-manta installed by default in Manta compute instances.

Thanks!

[trentm]

For the record: the internal "pony" ticket to have python-manta pre-installed in Manta compute jobs is MANTA-2207

[debrouwere]

:thumbsup: