Validate conditionally required default_pricing_plan_id in vehicle_types.json

From the spec, default_pricing_plan_id is:

```
REQUIRED if system_pricing_plans.json is defined. A plan_id, as defined in system_pricing_plans.json,
that identifies a default pricing plan for this vehicle to be used by trip planning applications for purposes 
of calculating the cost of a single trip using this vehicle type.
```