Resolve the branch name from the current repo branch
    
    We're following the branch naming convention here using
    c8-sig-storage-ceph-<ceph release> and instead of hardcoding
    the branch name in the script, we can resolve it from the
    current repo, making this Makefile portable (w/o changes)
    on the next branches (e.g.pacific).
    
    Signed-off-by: Francesco Pantano <fpantano@redhat.com>