build-vhost-mail: map to @vhost.o.b.e

This is a temporary fix for sandstorm delivery failures to @ocf.b.e. The
fix works by

- mydomain = ocf.berkeley.edu
- mydestination = vhost.ocf.berkeley.edu
  so only mail to vhosts stops here
- relayhost = smtp.ocf.berkeley.edu
  so everything not vhost is redirected to anthrax
- vhosts now map to user+abc@vhost.ocf instead of user+abc@ocf

This resolves rt#3080