Support corporate proxy via env var

Support routing outbound discovery traffic through an env var similar to how [update_engine](http://coreos.com/docs/cluster-management/setup/update-strategies/#updating-behind-a-proxy) works.

`HTTPS_PROXY`, `HTTP_PROXY` or `ALL_PROXY`

See http://stackoverflow.com/questions/25019355/coreos-fleetctl-list-machines-show-error