Greenwave conf and support for gating validation
    
    rpkg will validate the gating.yaml file if there is one in the repo. It
    will ask to Greenwave for that, when a build command is issued.
    The command will fail if the gating.yaml file is misconfigured. The user
    will have the chance to disable the check adding a parameter.
    With this change fedpkg will have the default configuration to query the
    Greenwave API.
    
    Signed-off-by: Giulia Naponiello <gnaponie@redhat.com>