RHEL may be RHN, but that's not default as of 6.3; try RHSM too

Code checked rhn-channel -l to find optional repo, but later RHEL
systems use the subscription-manager form of subscription, and rhn*
commands are not enabled.  Test first with rhn-channel -b, and if
that fails, check yum repolist instead (perhaps yum repolist would
work for RHN systems, too, but I don't have one of those).

Signed-off-by: Dan Mick <dan.mick@inktank.com>