Restore generic UI preamble text

The generic UI text in the policy base class was replaced with
the Red Hat Enterprise Linux wording in commit c59877c. The
wording isn't suitable for all distributions so revert this part
of the commit and move the RHEL text back to the RHEL-specific
policy class.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>