Allow chef-solo to use remote_file and other resources through proxy

Allows passing of http_proxy, https_proxy, and no_proxy variables to the chef-solo solo.rb file by adding the following variables to .kitchen.yml:
    chef_http_proxy: http://myproxy.com:8000
    chef_https_proxy: http://myproxy.com:8000
    chef_no_proxy: "*.internal.corp,10.*"