Add support for proxy environments variables (HTTP_PROXY/HTTPS_PROXY)

[barryib]

It would be nice to let this provider use proxy env vars. This will allow us to use this provider behind enterprise proxy.

I can open a PR to address this if needed.

[barryib]

FWIW, I just added in terraform-aws-modules#6 and it's working as expected.

[grahamhar]

@barryib any chance you can open a PR for this, I really need support for this too :)