allow sharing cache_store configs between database servers

(switchman handles the details)

test plan:
 * in cache_store.yml, have something like:

oneoff:
  cache_store: redis_store

development: oneoff

 * launch rails console in development mode; Rails.cache should be a redis store

Change-Id: Ie12e655264b1f8c4a5df6ef31cf84d12cc913eb6
Reviewed-on: https://gerrit.instructure.com/39923
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>