use REDIS_x_DB_INDEX instead of REDIS_x_DB, add optional setting to example env file

.env.example

@@ -28,10 +28,14 @@ MAX_STREAM_LENGTH=200
 REDIS_ACTIVITY_HOST=redis_activity
 REDIS_ACTIVITY_PORT=6379
 REDIS_ACTIVITY_PASSWORD=redispassword345
+# Optional, use a different redis database (defaults to 0)
+# REDIS_ACTIVITY_DB_INDEX=0
 
 # Redis as celery broker
 REDIS_BROKER_PORT=6379
 REDIS_BROKER_PASSWORD=redispassword123
+# Optional, use a different redis database (defaults to 0)
+# REDIS_BROKER_DB_INDEX=0
 
 # Monitoring for celery
 FLOWER_PORT=8888