Update /api/status response to match lorax-composer

[atodorov]

This is what we have in lorax-composer:

{
  "api": "0", 
  "backend": "lorax-composer", 
  "build": "19.7.14-1", 
  "db_supported": true, 
  "db_version": "0", 
  "schema_version": "0"
}

Original issue:
weldr/lorax#355