gracefully handle absence of cache_store.yml

test-plan:

 - don't have a config/cache_store.yml
 - try and boot canvas

Change-Id: I58ceca7bc3b618421e7d66987bbd18b2646babf4
Reviewed-on: https://gerrit.instructure.com/29582
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>