The change to move from `MemcachedCache` to `PymemcacheCache` broke things when upgrading from a Python 2-based Review Board 3.0.x to Python 3-based 5.0 beta 1. The problem is that various code paths during `django.start()` ended up triggering cache operations, and trying to set up the old (pre-migrated) `MemcachedCache` would fail due to a missing module. We migrate this setting during the upgrade process, but we never got that far. This change fixes rb-site to override any stored cache settings and just use the local-memory cache for the duration of the command. This is guaranteed to work regardless of what modules happen to be installed. Testing Done: Ran rb-site upgrade on a site where the old environment used the `memcached` package, and the new environment used the `pymemcache` package.