config: Don't reference internal PHP classes

[lazka]

Instead of

  payment_types:
    tuition_fee:
      service: 'Dbp\Relay\MonoConnectorCampusonlineBundle\Service\TuitionFeeService'

  payment_types:
    tuition_fee:
      provider: 'tuition_fee_provider'

where tuition_fee_provider is defined in the CO bundle config.

This way we don't expose PHP stuff to the config, which means we can change it later on, and is less confusing to the integrator.