ci: Use variables to build image names
ci: Use variables to build image names

This removes a lot of repetition and makes the configuration much
easier to read.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,crobinso/libvirt,jardasgit/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,crobinso/libvirt,crobinso/libvirt,jardasgit/libvirt