sanity check cache_store.yml and warn of misconfig

test-plan:
 - create a cache_store.yml that's not a hash
 - try to boot canvas, get an appropriate error
 - create a cache_store.yml with non-hash values for the top level keys
   (e.g. entire file is "cache_store: redis_store")
 - try to boot canvas, get an appropriate error

Change-Id: I4e8965a61c11c64e1d81894e8ae316761ac12f1b
Reviewed-on: https://gerrit.instructure.com/29663
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>