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.

Ran rb-site upgrade on a site where the old environment used the

memcached package, and the new environment used the pymemcache

package.