send the correct beta/test domains in LTI launches

test plan:
 - LTI launches should contain custom_canvas_api_domain
 - on beta the domain should contain .beta
 - on test the domain should contain .test
 - regression test all LTI launches to ensure they contain this custom variable

fixes PLAT-589

Change-Id: Ifb5b05d92633c97358166644978730b5af1ed062
Reviewed-on: https://gerrit.instructure.com/38902
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>