openshift.ks - add "EXTRA" repo vars for testing

enterprise bug 1119419
https://bugzilla.redhat.com/show_bug.cgi?id=1119419

This adds options for setting up additional repos for jbosseap,
jbossews, rhscl and rhel so we can test alpha/beta packages for channels
we depend on.
You can drop the `echo` and just use `> "${extra_repo_file}"` unless there is some reason to start with a blank line (in which case a comment would be helpful).