Added a firewall to servo-master1

[Josh Matthews]

Hi everyone! This is a heads up that I've enabled a firewall on
servo-master1 to limit the number of externally-visible services. I've
documented what I did manually at
https://github.com/servo/servo/wiki/Firewall-setup-for-servo-master1 ;
if you notice any surprising failures that involve build.servo.org or
other servo.org services, please file an issue with as much detail as
possible at https://github.com/servo/saltfs/issues/ .

Cheers,
Josh