Upgrade price model creation/update endpoint for specific service

[kowalczyka]

Summary
Currently when service have no price model defined, PUT endpoint is used to create it. The same endpoint is then used for updating price model. Using example for updating price model delivered with Swagger leads to an error.

Details
Price model update for given service and for given customer should be verified and upgraded if necessary. Proper description in Swagger should be provied so that client could use the endpoint without any issues.
Additionally issue #137 must be verfied and resolved

Acceptance criteria

• endpoints for setting the price model are working and are well documented
• Customer price model is created despite of incomplete request body #137 is resolved

Additional context
N/A

[kowalczyka]

#137 is not resolved, problem description was updated