Use constant time string comparisons for auth.
Use constant time string comparisons for auth.

Fix bug 942644.

Use constant time string comparisons when doing authentication to help
guard against timing attacks.

Change-Id: Iaaefb13f7618b06834630d9ccb97aff056b4bf4c

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects