uhd_images_downloader.py ignores HTTP proxy environment variables

[davidskun]

Issue Description

It appears that uhd_images_downloader.py does not read the values of the environment variables, HTTP_PROXY & HTTPS_PROXY, for setting the proxy global variable, _PROXIES, whose keys are 'http' and 'https'. Suggest updating the code to pre-populate the global variable, _PROXIES

Setup Details

Run current version of uhd_images_downloader.py behind a proxy

Expected Behavior

Setting the HTTP_PROXY & HTTPS_PROXY environment variables will download the FPGA images from Ettus.

Actual Behavior

_PROXIES is an empty dictionary so no FPGA images can be downloaded behind a HTTP/HTTPS/ proxy

Steps to reproduce the problem

Run uhd_images_downloader.py behind a proxy

Additional Information

None