default-network annotation is limited to just Multus namespace

The general gist is that the v1.multus-cni.io/default-network annotation is limited to just annotations in the multus namespace.

From this snippet:

	// The CRD object of default network should only be defined in multusNamespace
	networks, err := parsePodNetworkAnnotation(netAnnot, conf.MultusNamespace)

Found @:

multus-cni/pkg/k8sclient/k8sclient.go

Lines 606 to 607 in 9f5c023

    
           // The CRD object of default network should only be defined in multusNamespace 
        
           networks, err := parsePodNetworkAnnotation(netAnnot, conf.MultusNamespace)

Is this really the functionality that we want? I'd argue that it should follow the typical conventions for a selection annotation.

It's not in the documentation for it @ https://github.com/k8snetworkplumbingwg/multus-cni/blob/master/docs/configuration.md#specify-default-cluster-network-in-pod-annotations

	// The CRD object of default network should only be defined in multusNamespace
	networks, err := parsePodNetworkAnnotation(netAnnot, conf.MultusNamespace)