Improve local-config proxy handling.

We were applying proxy settings after apt-get install runs are done
in the Ubuntu element, which made it ineffective. Secondly, we were
not applying no_proxy settings to apt, which caused it to fail in
corporate settings. More explicit control may be facilitated in future
but local-config is a DTRT tool, and so we should help it DTRT.

I think I've correctly setup Zypper for this, and I have no idea
how to make the same improvement for Yum (and also Yum currently
doesn't set HTTPS proxies, which may lead to similar reports there).

Change-Id: Idbb9781d64d44b717ea22a25429383fa45d84148