RHEL7: Guide user to subscribe to "RHEL Server Optional" channel

python-cheetah requires python-pygments package that is only available
for RHEL7 system in the "RHEL Server Optional" channel at RHN Classic or
Red Hat Satellite. So guide user to subscribe to this channel in order
to install and run Kimchi.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>