When setting up rb-site's state, the cache is temporarily overridden

in order to have a place to stash settings and avoid polluting a

production server cache. Unfortunately, on a new install, this ends up

getting saved out to siteconfig settings, since the default stored

cache_backend comes from CACHES['default'].
We now explicitly set the desired cache backend when storing the initial

settings for the site.

Tested this with new local installs and Docker installs. Verified that

the configured cache setup was in use instead of a local memory cache,

and that this was reflected both in settings and in siteconfig.