Correct defaults for app center plugin

fixes PLAT-806

Change-Id: Iec9cb69c80fab471b1aff4d8b8d7bc2dc4190520
Reviewed-on: https://gerrit.instructure.com/45783
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Eric Berry <ericb@instructure.com>
Product-Review: Eric Berry <ericb@instructure.com>