mark cassandra gem integration tests pending if cassandra is not configured

Change-Id: I31e2d84345b650d1f0c073e740e31a674fcb6b6f
Reviewed-on: https://gerrit.instructure.com/29756
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>