Add a squid element.

From the Squid man page:

squid is a high-performance proxy caching server for web clients,
sup‐porting  FTP,  gopher,  ICAP,  ICP, HTCP and HTTP data objects.
Unlike traditional caching software, Squid handles all requests in a
single, non-blocking process.

We are considering using this as part of our CI overclouds
for caching.

Change-Id: I8eb006ab0af731587bfb0a99659f6dc8bed42fc7
Merge "Add a squid element."