list enrollment term api

fixes CNVS-10433

test-plan:
  - fetch /api/v1/accounts/<root account id>/terms
  - generate api docs and terms should render

Change-Id: Ic63197fccc21198922c8d737ff6fe6920f2e1d7c
Reviewed-on: https://gerrit.instructure.com/21610
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>