Make Gerrit port and dir selection deterministic

For every test number N run in process with PID, we take address
127.N.PID/256.PID%256 and let Gerrit listen on its default ports (29418
for SSH and 8080 for HTTP) on that address. We also name site dir with
the same address with "site-" prefix.

Thanks to Clark Boylan for this great idea.

Change-Id: I95b010e7a5b96c981614d5d3401efe861534806f