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>
Hmmm, another idea:  Completely drop this option, and test ``docker_repo_name`` value to decide.  For example:

In ``[docker_cli/build]`` section, set ``docker_repo_name = empty_base_image`` as the default setting, along with ``__example__ = docker_repo_name``.  Then, for tests that should NOT do substitution, you can override the option to empty with ``docker_repo_name =``.  That's super easy to test in code with ``if self.config['docker_repo_name'] == '':`` (all the values are stripped in parsing).

This will make documentation easier also, since every option documented in the subtest section, is automatically carried into (lower) sub-subtest sections (so you only have to document it once).  In other words, you can just put the option w/o documentation into any section and docs will be copied automatically from main section, when they build.