Support for Gitlab auth

[lorands]

As concourse supports Gitlab authentication (https://concourse-ci.org/install.html#auth-config) it would be nice to have Gitlab auth support as well.

[danyoung]

Hi @lorands thanks for your suggestion - we'd like to add this and will discuss it within our team

[crsimmons]

Even though the Concourse binary now supports gitlab auth, the BOSH release (which Concourse-up uses) does not. Even on master gitlab auth is not configurable.

[lorands]

Looks like, gitlab is now there: concourse/concourse-bosh-release@96460b2

[crsimmons]

Interesting. I guess that was an undocumented feature in Concourse 5.

I'll throw a story in our backlog but we have very limited engineering resources at the moment so we may not get to it for a while.

If you want to add it yourself feel free to make a PR and we'll take a look at it.