Validate OpenShiftSDN network type cannot be used with SNO clusters

Manoj noticed that the cluster registration fails for SNO clusters when the network type is set to OpenShiftSDN. We should add some validation to prevent this combination.

Failed to register cluster with assisted-service: AssistedServiceError Code: 400 Href: ID: 400 Kind: Error Reason: OpenShiftSDN network type is not allowed in single node mode

Documentation also indicates OpenShiftSDN is not compatible: https://docs.openshift.com/container-platform/4.11/installing/installing_sno/install-sno-preparing-to-install-sno.html