bad apt proxy config file in deployed nodes

In a local provider installation:

The host system contains the following:

/etc/apt/apt.conf.d/01proxy
Acquire::http::Proxy "http://localhost:8000";

The host system has squid-deb-proxy running so this apt-proxy configuration works as it should.

When you juju deploy an instance within the local provider, that same apt-proxy configuration parameter is placed directly in /etc/apt/apt.conf.d/42-juju-proxy-settings.

The problem is that the network bridge for a local provider using lxc is something like 10.0.3.1, however, the deployed node thinks the the proxy server is sitting at http://localhost:8000 so any attempts to deploy a charm fail due to apt not being able to install the required packages.

If I were to set apt-http-proxy in my environments.yaml file I could understand it being copied to the deployed node, however, the way juju seems to handle it now is just blindly copying over the proxy settings from an `apt-config dump`.

This type of scenario may have not been tested, however, this will put a damper on early users wishing to adopt juju and only having a single system to work with. This will also been seen a lot more since I believe a lot of users like to use squid-deb-proxy for their package caching.

The relevant code for this is in `utils/apt.go` and pretty much that entire package.