spec: canvas_cassandra vendored_gems cassandra setup

Change-Id: I2a5a2100f3f0d818376675f7a1e085032241a006
Reviewed-on: https://gerrit.instructure.com/36947
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>