nova: remove EPEL repository from nova-spicehtml5proxy image
nova: remove EPEL repository from nova-spicehtml5proxy image

The EPEL8 repository doesn't provide the necessary spice-html5 package,
and the image is marked unbuildable for CentOS. Let's not make EPEL look
more useful than it is.

TrivialFix

Change-Id: Ia37792ca6e5b40156ebd57b6c290d1ee9d4ff87a

openstack/kolla,openstack/kolla