Added doc about configuring juju to use proxies.

This new doc is a rewrite of the release notes (though maybe I need to update the release note with what I learned). I added examples and vetted them with Tim,

FIxes LP:1301964 and LP:1292167

Note that "Configure Proxy Access" is consistent with the names of the how to docs like "Mange" and "Deploy", but I see other sections use "Managing" and "Deploying". Don't hate me.