Add support for global `*_proxy` environment variables

Many programs (wget, curl, yum, etc.) can be configured to use proxies with `<protocol>_proxy` or `<PROTOCOL>_PROXY` environment variables. Based on the same environment variables on the host and Vagranfile configuration we'll generate  _/etc/profile.d/proxy.sh_ that will be evaluated by most shells.

## ![Under construction](http://www.animatedgif.net/underconstruction/5consbar2_e0.gif)

This PR is still work in progress. All comments welcome!