Add size validations to token controller.

Updates token controller so that it explicitly checks the max
size of userId, username, tenantId, tenantname, token, and password
before continuing with a request.

Previously, when used with the SQL keystone backend an unauthenticated
user could send in *really* large requests which would ultimately log
large SQL exceptions and could thus fill up keystone logs on the
disk.

Change-Id: Ie7e3a958829f99f080e66582bdf558cded70248c