Make sure GCE metadata requests don't get caught in / delayed by a proxy

[dhermes]

See googleapis/oauth2client#114 and googleapis/oauth2client#120

Issue with some context:
http://stackoverflow.com/q/11805773/1068170

[dhermes]

It seems likely we won't have an issue with httplib. The core issue is that urllib.request.ProxyHandler default behavior

is to read the list of proxies from the environment variables

and

to disable autodetected proxy pass an empty dictionary.

[dhermes]

Closing this out, most sure that this isn't an issue since our implementation uses httplib.

If it comes up (i.e. someone reports proxy issues on GCE or just proxy issues when checking for GCE), we can re-open.