settings/production: do not configure memcache if cache_uri is unset
settings/production: do not configure memcache if cache_uri is unset

this is needed so we can spawn a production-like webapp (in docker)
without a memcache (makes documentation writing easier).

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui