urllib3 1.21 fails kubernetes if you use configuration.assert_hostname

[mbohlool]

Look like the new version of urlib3 fails out tests:

logs: https://travis-ci.org/kubernetes-incubator/client-python/jobs/225699267

[mbohlool]

cc @dims

[mbohlool]

The change is urllib3 is explained in this issue: urllib3/urllib3#1016