Sanitizes authentication methods received in requests.

When a user authenticates against Identity V3 API, he can specify
multiple authentication methods. This patch removes duplicates, which
could have been used to achieve DoS attacks.

Closes-Bug: 1300274
(cherry picked from commit ef868ad92c00e23a4a5e9eb71e3e0bf5ae2fff0c)
Cherry-pick from https://review.openstack.org/#/c/84425/

Change-Id: I6e60324309baa094a5e54b012fb0fc528fea72ab