force the shard for User.of_account

fixes cross shard user search by id for rails 3

prevents the where from inferring the user's home shard, cause this query
really needs to happen on the account's shard

Change-Id: I0a3539be638ece276b8fb33f87c75e8a33ab51ef
Reviewed-on: https://gerrit.instructure.com/29589
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>