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>
Python will close the file when the variable goes out of scope, so no need for ``try...finally...`` here.  It's fine to let the ``open()`` call raise exception if file doesn't exist, then test should fail anyway.