dgit: read-only access to the Ubuntu Cloud Archive (Closes: #932322)

dgit: read-only access to the Ubuntu Cloud Archive (Closes: #932322)

This adds -d ubuntucloud for the Ubuntu Cloud Archive (see
https://wiki.ubuntu.com/OpenStack/CloudArchive for details thereof).

Access is by the aptget: query mechanism, suites are spelled like
bionic-updates:train or bionic-proposed:train, additionally the
shorthand bionic:train for bionic-updates:train is allowed.

Closes: #932322
Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>