Add 'pkgset_koji_builds' option to include extra builds in a compose
    
    This PR adds new pkgset_koji_builds configuration option.
    
    This option allows setting list of extra Koji build NVRs which will be
    included in a compose. This is useful in two cases:
    
    a) It allows generating standard composes with few packages update to
    certain version to test how the compose behaves when the package is
    updated for real.
    
    b) It allows generating compose consisting only from particular builds
    when pkgset_koji_tag = '' or None. This is useful when one want to
    regenerate the compose with packages which are not tagged in single Koji
    tag. This is very useful for ODCS when reproducing old composes.
    
    Merges: https://pagure.io/pungi/pull-request/1049
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>