Add new dbschema item for archive semantics

[Julian Edwards]

As part of the commercial repo work, we need a way of linking a distribution
to an archive via some context.

Make a DBSchema item that hardcodes the semantics -- so some tuples might
look like this:

  ubuntu-public /srv/archive.ubuntu.com/ ubuntu MAIN_ARCHIVE
  ubuntu-security /srv/security.internal/ ubuntu SECURITY_ARCHIVE
  ubuntu-commercial /srv/archive.canonical.com/ ubuntu COMMERCIAL_ARCHIVE
  ubuntu-obsolete /srv/obsolete.internal/ ubuntu OBSOLETE_ARCHIVE

(I called it "public" and not "main" archive to avoid confusion with the
main archive).

    - OBSOLETE_ARCHIVE could store warty and hoary, for instance.

    - SECURITY_ARCHIVE is published to a directory called .internal for
      good reason!

    - Are there other archives we'd like to have related to a distro?

This would still require some hardcoding of the behaviours in code but I
think that's a more flexible and simpler model than a full-blown
parametrized model. And we could have separate code modules to handle
what's specific in each of those cases.

[Julian Edwards (julian-edwards)]

I have rejected bug 119920, this bug will now encompass the work involved with that as it makes more sense to do it here. As part of this bug we'll now also create the DistributionArchive table, which a schema something like:

archive (int)
path (string)
distribution (int)
DistributionArchivePurpose (DBSchema enum, as above)

[Julian Edwards (julian-edwards)]

We're now not doing the new table; instead a new column "distribution" is being added to the archive table.

[Julian Edwards (julian-edwards)]

RF 4582