Make boot-stack's stackrc be metadata driven.

For now we're going to do credentials as Heat parameters, to break the
gordian knot of what-to-implement-first we're facing. As such the
in-instance credentials need to change to be source from Heat.
boot-stack in particular hardcoded 'unset' rather than consulting
metadata.

Change-Id: I35cc8d64fdc24d1fc57c0d0c25aabbe0eb83840d