Remove database URL defaults

[jnm]

1. Is it really useful to have defaults for database URLs? It's hard to imagine these existing for any reason other than to be overwritten, especially when we bizarrely have both localhost and redis_cache in the Redis defaults:

   kpi$ grep 'redis://' kobo/settings/base.py
   CELERY_BROKER_URL = os.environ.get('KPI_BROKER_URL', 'redis://localhost:6379/1')
   'REDIS_SESSION_URL', default='redis://redis_cache:6380/2'
   'default': env.cache(default='redis://redis_cache:6380/3'),
   

   1. The non-localhost hostnames look like they were set to match kobo-docker service names. If kobo-docker really depends on the default values, which I doubt, then let kobo-docker handle setting the environment variables appropriately.
   2. Removing the defaults would make it efficient to tell when the databases are not accessible, but that wouldn't differentiate between configuration errors and "non-runtime" mode.

Originally posted by @jnm in #3820 (comment)