COVERITY: Add coverity support
    
    Using travis-ci we can start doing coverity scans on every pushed code.
    This is not something new as so far we have been relying on sgallagh's
    internal infra to do so, unfortunatelly the infra is about to be
    retired ... thus, start to use public coverity's instance is a hard
    requirement for us.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Signed-off-by: Edjunior Machado <emachado@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>