Use server-side GitLab Runner description as default value when registering runner

The following discussion from !4158 (merged) should be addressed:

• @vshushlin started a discussion: (+6 comments)

  Thanks, @pedropombeiro!

  I left a couple of non-blocking questions for you. 🏓

  Feel free to pass this to the maintainer next time, if you decide not to change it.

Proposal

Reach out to GitLab to request runner details once we know the runner ID from POST /runner/verify to get to know the description. Use that as the default value for the name.