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>
Not sure what this means, it sounds like a config. option you're not suppose to change (meaning, how is it an "option" then?).  I would suggest instead to remove the option from this section, and have the code do something like ``if 'dockerfile_use_config_repo' not in self.config:`` or similar logic, to detect if substitution should be totally bypassed.