Add size validations for /tokens.

Updates /tokens 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: Ic7f9291b6020f2579ec55431358af16eb2ebb66f
Merge "Add size validations for /tokens." into stable/essex