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.

Change-Id: Iec9a1875a4ff6e2fac0fb2c3db6f3ce34a5dfd1d
Closes-Bug: 1300274
Merge "Sanitizes authentication methods received in requests." into milestone-proposed