Add a new squid3 module and replace in-grown use

install-server::caching-proxy and url_downloader both install &
configure squid3 and are *very* similar to each other (in fact they seem
derived from one another, then diverged a bit).

Abstract away the squid3 bits into a simple squid3 module that accepts a
configuration file or template and use it from the two modules.

The new module doesn't have rspec tests but the callsites do. This
should probably be fixed, by someone more knowledgeable in rspec, like
the author of both of these other two modules *cough*.

Change-Id: I86377761019fb65106ce56e99d8ba39f1dc4b10b