builder should also support password callback

[Cryptophobia]

From @smothiki on May 11, 2016 22:57

Right now we are just using password callback in tests, it would be nice to support password call back , keeping in mind stand alone builder work.

Copied from original issue: deis/builder#331

[Cryptophobia]

From @bacongobbler on October 31, 2016 20:11

I don't think this is something we want to allow. In a multi-tenant environment everyone should not have access to the git user's password, as there is no way we can authenticate who is pushing the build in the controller.

Keeping this open for discussion but I believe we should only allow keys. Much easier to allow/reject than changing a global password on a public PaaS.