add a service parameter to our CAS logout

fixes CNVS-12161

test plan
 - regression test CAS login and logout
 - logout should now include service parameter

Change-Id: Ieab7d53021eaf8697bf1636a438a6da4556df905
Reviewed-on: https://gerrit.instructure.com/33498
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>