role::cache::upload now uses varnish on labs

Turns out installing squid on beta is broken beyond repair:
 - the Lucid version of puppet choke on invalid syntax in our puppet
   manifests.
 - on Precise the WMF squid packages depends on libdb4.7 which is not
   provided under that distribution.

There is an existing hack to get upload cache to use Varnish instead of
squid. This patch reuse that hack to get Varnish on labs.

(bug 45122) migrate all beta instance from Lucid to Precise

Change-Id: If79da7686402a4f6da69eba2d157a449f2c8d3a1
Merge "role::cache::upload now uses varnish on labs" into production