implement encryption_key settings for rails 3 switchman shard

Change-Id: I9637715c285890404d22d1f78c07def3ca7930e1
Reviewed-on: https://gerrit.instructure.com/29271
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>