set ENV vars for http proxies
percent encode @, : in proxy user/pass
test proxy env vars available for shell_out
rename configure_environment_variables to configure_proxy_environment_variables
update for setting proxy env vars
use URI to parse/build proxies
add error handling, set *_proxy instead of *_PROXY
restore ENV after tests
update error message
fix indenting
Merge pull request #1459 from opscode/CHEF-4712

set ENV vars for http proxies