Specify cluster default network with a new CRD object.

[pliurh]

In additional to #131 and #153 , I think it would be better to specify the cluster default network with a CRD object instead of config files. My idea is to create a new CRD object. It may looks like:

apiVersion: "k8s.cni.cncf.io/v1"
kind: MultusDefaultNetwork
metadata:
  name: default
spec: 
  networkAttachememntDefinition: default/calico

Multus can get the name of default network from this CRD object, then retrieve the CNI config from that networkAttachememntDefinition object.

There are some advantages I can think of:

1. When administrator want to switch the default network from one to another, he doesn't need to update the file on each node.
2. It ensures consistency of the default network config across the cluster.
3. It can be accessed though kube API.

[pliurh]

@s1061123 @dougbtv I wonder if this issue can be re-open as a Feature Request. As this would be helpful for some scenarios like cluster network migration, where user can switch the cluster default CNI through the customer resource instead of updating multus.conf file on each host.