This region is currently timing out all instance boots. Zuul and
nodepool eventually hand this work off to other regions but only after
iad gets 3 attempts which can take at least 15 minutes. This slows down
jobs starting if the attempt to get nodes from iad.
Set max-servers to 0 so that we stop attempting boots there. OpenDev
admins can debug this out of band using manual boot attempts to figure
out when it is safe to reenable the region.
Change-Id: I794d6298ec9bb725075dde76c8549fe244c0599e
0676e2750a