Only set priority of rhel-server-ost-6-folsom-rpms if it exists

We were setting this before subscribing with subscription-manager and
even if rhel-server-ost-6-folsom-rpms didn't exist, this commit moves
the order around a bit and test for the existence of
rhel-server-ost-6-folsom-rpms
https://bugzilla.redhat.com/show_bug.cgi?id=904670

Change-Id: I698f7acbfbadeb13167173565467978b9e1683ec