Since the new maps style is delayed, we need to start working on the migration to stretch with the current style.
* prepare puppet change to isolate maps2004 from current cluster
* reduce cassandra replication to allow running on a 3 nodes cluster
* depool maps2004
* install new disks in maps2004
* reimage maps2004 as stretch, with new partition scheme
* initial OSM import on maps2004
* regenerate tiles on maps2004
* wait tile generation (probably a long, long time)
* validate that everything looks good
* depool maps2003
* reimage maps2003 as maps slave
* increase cassandra replication on the new cluster
* depool maps200, pool maps200
* since this upgrade does not bring a new style, the tiles generated by both new and old install should be equivalent, having tiles served from both stretch and jessie, from the same DC or from different DC should not be an issue.