Skip running rhn autoinstall script when it's already registered

Don't run the rhn autoinstall script on upgrades or in situations
where the config files already exist, under the assumption we've
already registered

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1045990
Change-Id: I409181a8e894c865f5597cb92265196006271e37
Signed-off-by: Joey Boggs <jboggs@redhat.com>