Add cernvm wpad and backup proxies

Make a cernvm-wpad service that returns one of these things in this order:
1. If at a recognized grid site: the WLCG squids + cvmfs backup proxies.
2. If there have been more than N requests from the same ISP during the last M minutes, return cernvm backup proxies. Make N and M configurable, but perhaps 100 in 60 minutes is a good first guess.
3. Otherwise, return Bad request. CernVM will be configured with auto;DIRECT so it will then go DIRECT to the openhtc.io servers.

In this way, people running large numbers of cernvm processes in a cloud will not hit the openhtc.io servers in large numbers and we will be able to detect them.

Use the 4th squid service (port 3125) and the cernvm-wpad name for the backup proxies.