When on a tag, trigger a multi-project pipeline in the CNG repostiory.

Opting for a trigger rather than an addition to our release-tools
project for a few reasons:

- The Dockerfiles in the CNG image repo change infrequently, and as a result
I don't feel the need/overhead for stable branches in that repo at this time
- My intent with the CNG repo, is that once stable, the Dockerfiles
would actualy move to their component projects, to be versioned with the
code they are building
- It is likely that we will want to followup with a manually triggered package
for branches for devs, and possibly review apps, so it made sense to
build the CNG ci jobs to accept this sort of pipeline.