Avoid possible timing attack in metadata api

Introduce a constant time comparison function to
nova utils for comparing authentication tokens.

Conflicts:
	nova/tests/test_utils.py
	nova/utils.py

Closes-bug: #1325128
Change-Id: I7374f2edc6f03c7da59cf73ae91a87147e53d0de
(cherry picked from commit 9f59ca751f1a392ef24d8ab73a7bf5ce9655017e)