Fix SSL error when connecting from behind a proxy

In certain versions of python, urllib fails when making https requests
from behind a proxy. See http://bugs.python.org/issue1424152 for
details.

This patch fixes an issue with pyVim by using urllib2 instead of urllib,
a change similar to existing code (see OpenUrlWithBasicAuth() and
OpenPathWithStub()).
Merge branch 'fix-ssl-behind-proxy' of github.com:osu-sig/pyvmomi into fix-ssl-behind-proxy