gitUrl support for Gerrit

[spmiller]

Our local Gerrit server has disabled HTTPS clone operations. We have to use SSH to clone, and Gerrit has a helpful link to do this in the web UI in the General tab.

I would like the existing Gerrit platform support to read the server config to see what download schemes are supported, and use ssh if configured by the existing gitUrl parameter.

I have a fork that has implemented this, which seems to work well: https://github.com/spmiller/renovate/tree/feat-gerrit-gitUrl

I can see that @felipecrs has some Gerrit WIP, and I don't want to step on any toes. Is this change something that would be suitable for a PR?