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>