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

Closes #1583.
Sure, we could. I guess I was looking at `auth:cancel` which doesn't allow this on the command-line. But since `auth:login` and `auth:register` do, we should make these available as flags too.

I did include a `gexpect` go test to test the interactive prompts, like we do with `auth:cancel`.