Avoid possible timing attack in metadata api

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

Conflicts:
	nova/utils.py

Closes-bug: #1325128
Change-Id: I7374f2edc6f03c7da59cf73ae91a87147e53d0de
(cherry picked from commit 4a60c6a655006b2882331844664fac5cf67c5f34)
Merge "Avoid possible timing attack in metadata api" into stable/icehouse