Sanitizes authentication methods received in requests. (bnc#873127, CVE-2014-2828)

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 e364ba5b12de8e4c11bd80bcca903f9615dcfc2e)
Cherry-pick from https://review.openstack.org/#/c/84425/

Change-Id: I6e60324309baa094a5e54b012fb0fc528fea72ab

Conflicts:
	keystone/auth/controllers.py
	tests/test_v3_auth.py