Use version-set in relative location

This makes version-sets work out of the box for NID repos, which is pretty cool! You can simply mirror a repo from gitlab.com/nso-developer/ to your local Gitlab environment, like gitlab.example.com/acme/a-package and it will automatically find the version set in gitlab.example.com/acme/nso-docker/ at the path /version-sets/acme/

There is one extra step for setting things up, since when someone first sets up nso-docker in their own environment, they now will need to create a version-set. However, since you are meant to only test with the NSO versions you are interested in, this should be done anyway.

Closes #251 (closed)