Fix build case + provide repo replace interface

1.Fix the build fails_msg
The commented line is for 'empty_base_image' and another one is based the rhel7 image
2.Provide the repo replace interface
Use two functions to find the Dockerfile's FROM token and replace its value which is what the repo is.
And provide the configuration dockerfile_use_config_repo to control it

Signed-off-by: Time Su <lsu@redhat.com>
Small nit-pick: I don't like private methods (name starting with ``_``) in base-classes if they do very general work.  Better to leave them with a public name.  Unless it's really doing something very specific, or could be dangerous if called from ousdie base-class.  It is small personal-preference thing though, so no requirement to change it, what you have is fine.