Move gerritbot to a less sensitive server

[Jeremy Stanley]

There is no real need for Gerritbot to be on the Gerrit server, so it should move elsewhere. That server hosts our authoritative VCS copies and enforces the access controls for the entire release process, and we should be trying to run as little else there as possible.

The eavesdrop server might be a good alternative, since it already hosts a couple other bots (meetbot, statusbot)... though there is an argument to be made that meeting logs and vote tallies count as sensitive data, and that we should be making an extra effort to secure those further as well.