webproxies service entries in wmnet.

We have two web proxies (brewster and carbon) to reach the outside
world. Since the machine could change, this patch introduce two new
service entries:

 webproxy.pmtpa.wmnet (brewster)
 webproxy.eqiad.wmnet (carbon)

This would let scripts and Jenkins jobs refer to a stable hostname
regardless of future servers changes.

bug: 59253
Change-Id: I60b3e58a1bb7050758b8851cb6eafa1113352e74