Support proxy for downloads

[colinjoy]

Using dlite 1.1.5, the installation fails at downloading the OS.
Assuming the proxy is ignored?

$ sudo dlite install -d 4
Building disk image: done
Downloading OS: ERROR!
Get https://github.com/nlf/dhyve-os/releases/download/v2.3.0/bzImage: dial tcp 192.30.252.120:443: i/o timeout

$ wget https://github.com/nlf/dhyve-os/releases/download/v2.3.0/bzImage
Resolving proxy...
Proxy request sent, awaiting response... 200 OK
Length: 3277392 (3.1M) [application/octet-stream]
Saving to: 'bzImage'
bzImage                       100%[=================================================>]   3.12M   664KB/s   in 5.1s

$ echo $http_proxy $https_proxy
http://proxy:8080 http://proxy:8080

[nlf]

correct, the downloads currently do not respect proxies. we should definitely fix that!