updated CACHE_BACKEND to CACHES of dashboard's local_settings.py

Fixes bug 1194220

Cherry-picked from https://review.openstack.org/34651

Change-Id: I09ea8650962c0ed33ce819bf1d620d8bb3694a83
Merge "updated CACHE_BACKEND to CACHES of dashboard's local_settings.py" into stable/grizzly