Support setting *_proxy env vars for Vagrant on the host

[tmatilai]

It might be useful for having a config option for setting HTTP_PROXY etc. environment variables on the Vagrant run. This could maybe even be the default on "local" providers like VirtualBox (especially with #38), while normally doesn't make sense with cloud providers.

The variables would set proxy for Vagrant's downloader which is used for downloading boxes, Chef's "install.sh". But wouldn't help with vagrant plugin install as plugins are not loaded by vagrant plugin commands.

Ref: #52 (comment)

[johnbellone]

@tmatilai What do you mean by setting them on the Vagrant run? I set them at the beginning of the run using Dotenv.load and it seems to work throughout the provisioning.

[tmatilai]

I planned on setting the env vars when the plugin is loaded. But not overriding existing variables. Or maybe to hook to environment loading as there is no hurry to set the variables.

Using dotenv or just manually setting them in Vagrantfile has the same effect. But Vagrantfile is not evaluated with vagrant plugin and (maybe?) vagrant box commands. And it is not clear nor documented that users need to set the variables in the first place so it would help if we could do it automatically for vbox etc. providers.