compose: Add a --add-override-pkg-repo argument

As a developer, a workflow I have for testing things is to create
an RPM, toss it into a local yum repository, then do a compose.

However at the moment to add the local overrides I have to edit the
treefile, which is annoying.  Let's add a commandline override for
this.

Note this also deletes the old "repos_data" code which was not being
used.