add fixed and variable parameters for lti2

fixes: PLAT-593 PLAT-594

test-plan:

specs should pass

Change-Id: I05e9b408655779f1c53ef17d5c98aeaf2a8e3a87
Reviewed-on: https://gerrit.instructure.com/39409
Reviewed-by: Brad Humphrey <brad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>
QA-Review: Nathan Mills <nathanm@instructure.com>