Enable usage of proxies defined in environment (http(s)_proxy).

As far as proxies usage is concerned, keystone-client API and swift-client API behave differently because the former uses python Request library while the latter uses raw httplib. As a result, Keystone authentication honors environment variables http_proxy, https_proxy and no_proxy while Swift doesn't.

This patch, which code is mainly borrowed from Python Requests, makes Swift
data connections and Swift authentication connections behaving homogeneously.

Change-Id: Ic8a0089c35c458d7ed96e572e22429014298fe4c
Merge "Enable usage of proxies defined in environment (http(s)_proxy)."