Restructure replication in preparation of moving off manganese

Ytterbium now has the gerrit software running, and replicates to
all the destinations that manganese used to. Manganese is still
acting as master and will continue to do so until we cut over.

At that point, switching to the new server should just be a DNS
change.

Change-Id: I187e39e0d51bc1ec95119a47114e2f8755471157
Merge "Restructure replication in preparation of moving off manganese" into production