Respect no_proxy environment variable

[davidwegs]

Respect no_proxy environment variable

My corporate network enforces proxy use during HTTP/S connections through the http_proxy/https_proxy environment variables. However, we typically set the no_proxy variable to avoid hitting the proxy for internal connections, including connections to many cloud resources.

Currently libcloud respects https_proxy by setting the proxies dictionary directly in the request library's Session object. This seems to override the library's usual handling of no_proxy. I propose a small change to respect no_proxy as well, which will provide a consistent experience for libcloud users.