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>
I made a mistake here, the cases use url will fail here   if not os.path.isfile(file_path):  ,sorry about it. I have a fix locally, but the finally method  depends on how to handle the way how to handle the "Change config repo " issue, https://github.com/autotest/autotest-docker/pull/430/files#r21894310 here.