Make sure we configure the proxy before doing apt-get update.

[tbroyer]

In the case your machine can't access the internet except through a proxy, you want it to be set before you do the apt-get update!

I used a notify dependency just in case you forgot to set the proxy in the first place (or mistyped it) and thus had the apt-get update failing. Without the notify, the apt-get update wouldn't get executed after you fix the proxy issue, as the sources.list and sources.list.d would have been set in the first (failing) run (I mean, unless you use always_apt_update, but that's another story).

I didn't set a dependency on the python-software-properties package because:

1. I believe it should be moved to apt::ppa (see Move Package['python-software-properties'] to apt:ppa #40)
2. you'll probably have something like this defined: Class['apt'] -> Package <| |> which would then guarantee the apt-get update will be executed before you install any package