Can't add default backend with Ingress resource

[TsvetanMilanov]

Currently the only way to add default backend configuration is through the ingress-merge ConfigMap. It will be useful to be able to define the default backend in Ingress resource. This will enable the following scenario:

1. Create ingress-merge ConfigMap without default backend.
2. Deploy service A with Ingress resource.
3. Test service A.
4. Deploy service B which uses service A and which adds the default backend through Ingress resource.
5. Test service B.

Currently service B should be deployed before the ingress-merge ConfigMap is created and before service A is deployed. If service B can add the default backend in its Ingress resource, the ingress-merge controller will support the scenario described above.

The feature proposal is:

1. Keep the current workflow.
2. Respect default backend configuration from Ingress resource.
3. Fail if there is default backend in the ingress-merge ConfigMap and in some Ingress resource which is configured to use the ingress-merge ConfigMap.
4. Fail if two or more Ingress resources configured to use one ingress-merge ConfigMap have default backend configuration.