Use gitlab.com for the centpkg-sig
    
    Previously broken cloning has been fixed in 875769 & 8e123d and now
    expects the repos to be present on the gitlab.com/CentOS/‹SIG›/rpms
    namespace, given the validation happens against gitlab.com it doesn't
    make much sense to expect the repos to be present on git.centos.org.
    
    Therefore change the default dist-git URLs for SIG repos to be on
    gitlab.com rather than git.centos.org
    
    Signed-off-by: Matej Focko <mfocko@redhat.com>