Update Respondus SOAP endpoint to use Quizzes NS

Since the namespacing of Quizzes, their "asset_strings" have become
"quizzes:quiz" instead of just "quiz" which was causing this endpoint to
not locate the quiz objects from the content migration.

Closes CNVS-11732

TEST PLAN
---- ----

See https://www.pagehub.org/canvas/quizzes/tickets/cnvs-11732#test-plan

Change-Id: I97369f9c50881dc1c6541fc0a824d9f332c99e0e
Reviewed-on: https://gerrit.instructure.com/41905
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>