[Beaker] Add RHEL-7.9 definition
[Beaker] Add RHEL-7.9 definition

So far, image reference for 7.9 is missing from IR's Beaker plugin vars.

RHOSINFRA-3920

Change-Id: I3acc8fec2eee7f1852910f22c1986b2202e040be

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared