Note about need request body json props in mgmt api openapi spec (MgmtRolloutRestRequestBodyPost)

[fkromer]

Responses of POST /rest/v1/rollouts would be less misleading if we'd provide a note about the need for either amountGroups or for groups in the MgmtRolloutRestRequestBodyPost schema. Alternatively setting a default value for amountGroups (groups the rollout should split targets into) of 1 would be reasonable as well.

{
    "exceptionClass": "jakarta.validation.ValidationException",
    "errorCode": "hawkbit.server.error.repo.constraintViolation",
    "message": "Either 'amountGroups' or 'groups' must be defined in the request"
}