rpmdefines changes depending on rpkg
    
    Recent changes on rpkg side require modification in centpkg too.
    rpmdefines in rpkg had to be changed because of a safer way
    of executing the 'rpm' command in the 'subprocess' python library
    (without shell=True argument).
    A related change in rpkg: https://pagure.io/rpkg/pull-request/620
    Other tools depending on rpkg (rhpkg, fedpkg, centpkg, rfpkg) are
    affected.
    Example for fedpkg: https://pagure.io/fedpkg/pull-request/486
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>