add IDS_PER_SHARD stub

test plan
 - run migrations with just canvas-lms
 - migrations should not fail

Change-Id: I489fd5c89457771ae2d21a7d110274d3843b3062
Reviewed-on: https://gerrit.instructure.com/31264
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>