propagate proxy settings to VM, docker container and test run if set

* By setting http_proxy in the host, vagrant can download the box but fails to download & install docker. Hence the change to Vagrantfile.
* See here why the vagrant reload is required: https://github.com/tmatilai/vagrant-proxyconf/issues/97.
* Finally, proxy settings need to be propagated into the VM for ```curl https://storage.googleapis.com/golang/go1.3.3.linux-amd64.tar.gz``` to succeed.