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.
Here you use `[ "${foo}x" != "x" ]`; elsewhere, you use `[ "x${foo}" != "x" ]`.  I doubt anybody will really notice, much less care, though.