bots: Switch rhel-7-7 image to stable repository
bots: Switch rhel-7-7 image to stable repository

RHEL 7.7 was released, so build that from the stable instead of nightly
repository (the latter will go away soon).

Prepare rhel.setup for RHEL 7.8. This does not exist yet, but will avoid
having to dig in git history once we build images for it.

Closes #12506

cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpituous/cockpit,cockpit-project/cockpit,garrett/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,deryni/cockpit,garrett/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,martinpitt/cockpit,mvollmer/cockpit,garrett/cockpit