bots: Move centos-7 image to virt-install-fedora
bots: Move centos-7 image to virt-install-fedora

This builds centos-7 the same way we build all RHEL images.

Let's deprecate virt-builder-fedora and eventually drop the libguestfs
dependency.

Closes #12549

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