docs need to have page explaining proxy configuration

[Nick Veitch]

Support for proxies

Proxies can now be configured for the providers in the environments.yaml
file, or added to an existing environment using "juju set-env" The configuration options are:

    - http-proxy
    - https-proxy
    - ftp-proxy
    - no-proxy

The values that are set for these proxies are exported in all hook
execution contexts, and also available in the shell through "juju ssh"
or "juju run".

There are three additional proxy options specific for apt. These are set
to be the same as the non-apt proxy values, but can be overridden independently:

    - apt-http-proxy
    - apt-https-proxy
    - apt-ftp-proxy

For example, with a squid-deb-proxy running on a laptop, you can specify the apt-http-proxy to use it for the containers by specifying the host machine’s network-bridge:

    apt-http-proxy: http://10.0.3.1:8000