validate format of external tool id

fixes CNVS-3055

Change-Id: I2e3394245573ff5b59ccb3012618f73754d166e8
Reviewed-on: https://gerrit.instructure.com/29544
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Bracken Mosbacker <bracken@instructure.com>