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.
I think it's acceptable; if you're using the "EXTRA" repo options, you'll know what "rhel_extra" is when you see it in repolist or whatever.