zun: Deprecate Zun provisionally

* Zun currently has a dependency on a combination of Docker and
  etcd that is no longer installable (or supported) in newer
  distros.
* These components are core dependencies of other systems and
  vendoring them will be very painful.
* Work to update Zun past this is scheduled for the 'C' cycle.
* If Zun regains compatibility, backports to this series will be
  considered.

Change-Id: I6a2a4ffdcaf1c4fab1097e94d5f255ffb9a2947d