Allow enabling documents that only appear as part of a translation

[ghost]

Via doc-suse-com/site#49 : It would probably help to be able to define additional documents for each language. There is no currently known case where we need that, setting prio accordingly.

This could be done by allowing something like deliverable@type=translation-only:

      <language lang="es-es" translation-type="list">
        <branch>maintenance/SLE15SP3</branch>
        <subdir>l10n/sles/es-es</subdir>
        <deliverable>
          <dc>DC-SLES-installation</dc>
        </deliverable>
        <deliverable type="translation-only" category="installation-upgrade">
          <dc>DC-SLES-jeos</dc>
          <format html="1"/>
        </deliverable>
      </language>

I have not quite thought through pros/cons here, but I think we should be cautious not to encourage people to add duplicative configuration. The DCs referenced in such elements must be different than those mentioned for the default language.