fix nova test_enforce_http_true unit test

When the nova policy patch was merged, causing
nova.tests.test_policy.PolicyTestCase.test_enforce_http_true to fail
randomly in the gate. The oslo has used the six.moves.urllib.request
instead of urllib2, but the patch didn't sync this in test_policy.

This related commit a88d9d5936aabc52046ed3ae566741422bfaed78.
Related-Bug: #1288178

Change-Id: I51b6bbf37b062edcf4c4b5f65adb8bfecd4a2eef
Merge "fix nova test_enforce_http_true unit test"