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>
Ahh, be careful here, this might be a bug:  For your own knowledge- from ``[DEFAULTS]``, we have fully-qualified-image-name (FQIN) broken down into for components.  To get the full name (FQIN) from the parts, do ``fqin = dockertest.images.DockerImage.full_name_from_defaults(self.config)``.  This combines values from options  ``docker_repo_name``,  ``docker_repo_tag``, ``docker_registry_host``, and ``docker_registry_user``.  So if in customizing this test, someone removes this option (so it uses the default), it will *not* be the expected full name.  I am sorry if this was not made clear in comments / documentation, it is my fault.

For this test, maybe there is easy fix.  Just rename this option to, for example ``base_repo_fqin`` instead.  Then there will be no confusion vs. the ``[DEFAULT]`` options, and name shows intention to put the entire FQIN all in one.  So this will make documentation easier as well.