Infra: maintenance overnight on 18-19th Feb

Rackspace are carrying out some network maintenance in the zone
containing the majority of our servers tonight, from 23:00 EST on 18th
Feb to 03:00 EST on 19th Feb. They expect network downtime to be 30-60
seconds per server, so probably unnoticeable.

A few of our servers have external block storage (Jenkins master, web
server and one of the Debian builders), so they'll have a temporary
disruption too.

I'll check on services at the end of the maintenance period, and please
reply here if you see a service problem at this time.

ยทยทยท -- Dominic Cleal