feat(controller): add "deis auth:passwd" to update password

Closes #1583.
I had originally written it as PATCH, and I can go back there if you would like. We're not really doing proper REST here with `auth` IMHO anyway--we aren't really doing CRUD operations on a User model, we're just calling some verbs like "passwd" and "login". Also it would have been the only PATCH in the API.

Your call--it's a simple change.