git-gerrit-mirror: elaborate project list handling

To support automatic mirroring of new projects, git-gerrit-mirror needs to get a list of projects from upstreams. Initially, that was hardcoded to be done via gerrit ssh interface. But later AOSP banned SSH access, and series of workaround were applied to deal with that. Those workarounds are limited-use and confusing (essentially, they require running mirror upstream-by-upstream, as they params globally). That should be fixed, and project list related params to be set on per-upstream basis.