While testing https://bugs.launchpad.net/landscape-client/+bug/1531150, I verified that the package-changer also didn't get the proxy setting from /etc/landscape/client.conf.
I followed the test procedure from that bug, and while the reporter used the proxy just fine, when it came down to actually install packages, the changer failed:
FetchFailedException: Failed to fetch http://br.archive.ubuntu.com/ubuntu/pool/main/p/perl/libperl5.22_5.22.1-9ubuntu0.2_amd64.deb Cannot initiate the connection to br.archive.ubuntu.com:80 (2801:82:80ff:8000::5). - connect (101: Network is unreachable) [IP: 2801:82:80ff:8000::5 80]
Failed to fetch http://br.archive.ubuntu.com/ubuntu/pool/main/p/perl/perl_5.22.1-9ubuntu0.2_amd64.deb Cannot initiate the connection to br.archive.ubuntu.com:80 (2801:82:80ff:8000::5). - connect (101: Network is unreachable) [IP: 2801:82:80ff:8000::5 80]
(...)
I then configured the proxy in /etc/apt/apt.conf.d/90proxy, restarted the client, and then the changer was able to download and install the packages.
So while there is a workaround like the above, since we have the proxy explicitly configured in /etc/landscape/client.conf, it should be used by processes spawned by landscape without the need for another configuration elsewhere.
