add custom role variable substitution

Test Plan:
 - custom variable $Canvas.membership.roles should return all roles
   including custom roles

Change-Id: Ib761d0f034de5431a51fa284401686e3269861e9
Reviewed-on: https://gerrit.instructure.com/37432
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>