Webhook accepts invalid service tags

[huib-portainer]

Is your feature request related to a problem? Please describe.
When using the service webhook with an invalid tag the status of the swarm service’s task becomes “rejected” and the swarm service becomes out of service.

e.g.
curl -X POST http://localhost:9000/api/webhooks/5d300dae-8ab8-4ce5-b429-cd82735de521?tag=hahaha

Describe the solution you'd like
The service webhook should guard against invalid tags