Make ubuntu templates squid-deb-proxy-client aware

This makes the ubuntu and ubuntu-cloud templates automatically aware of apt
proxy settings when the LXC host has "squid-deb-proxy-client" installed. This
makes installations *much* faster when a suitable squid-deb-proxy is
found on the network (or installed on the host).

Signed-off-by: Chris Glass <tribaal@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>