add Poll#associated_shards

This commit adds the Poll#associated_shards method so that
with_each_shard acts sane.

Test plan
- The poll_choices index endpoint should work correctly

Change-Id: Iabb0f3db25aec7e60ec1013424b534fa2c08102a
Reviewed-on: https://gerrit.instructure.com/36068
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>