Juju's proxy settings (http-proxy, https-proxy and ftp-proxy) are used to set proxy behaviour within the machine and unit agents and are also injected into the profile of the ubuntu user (see ~ubuntu/.juju-proxy). This means that software that is run within hooks or via juju ssh sees the correct proxy settings but software running in other contexts won't.
The proxy settings should really be set globally, via /etc/profile.d/ and not just for the ubuntu user.
A nice side-benefit of this is that the jujud agent will be using proxy settings that are likely to be correct from the moment it starts, instead of having to wait for the proxyupdater worker to start and take action.
To remember: for the manual provider, the file left in /etc/profile.d/ should be removed if a machine is no longer managed by juju.
Ideally, I'd love it if these were set in a global systemd location with DefaultEnvironment= so that when a charm installs a systemd service the systemd service automatically has the proxy environment variables set from juju.
Bonus if the equivalent is done for upstart for trusty for older charms.