Don't catch satellite with subscription-manager

Use rhnreg_ks for registering to satellite even on EL7 as long as
the certificate name doesn't end with .pem

Change-Id: I7d59dc07855da8b99e742b6ed3f8482d5cd5e5fa
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1166110
Signed-off-by: Ryan Barry <rbarry@redhat.com>