[BUG] updates to an existing attachment should be rejected

[maiqueb]

Describe the bug
An update to an existing attachment is currently being accepted, and treated as the following sequence of operations (in this order):

• add new interface
• remove old interface

Expected behavior
The controller should reject this operation, logging this.

To Reproduce
Steps to reproduce the behavior:

1. provision a net-attach-def + pod with network selection elements using it. This net-attach-def should not specify the interface name.

---
apiVersion: "k8s.cni.cncf.io/v1"
kind: NetworkAttachmentDefinition
metadata:
  name: macvlan1-config
spec: 
  config: '{
            "cniVersion": "0.4.0",
            "plugins": [
                {
                    "type": "macvlan",
                    "capabilities": { "ips": true },
                    "master": "eth1",
                    "mode": "bridge",
                    "ipam": {
                        "type": "static"
                    }
                }, {
                    "type": "tuning"
                } ]
        }'
---
apiVersion: v1
kind: Pod
metadata:
  name: macvlan1-worker1
  annotations:
    k8s.v1.cni.cncf.io/networks: '[
            { "name": "macvlan1-config",
              "ips": [ "10.1.1.11/24" ] }
    ]'
  labels:
    app: macvlan
spec:
  containers:
  - name: macvlan-worker1
    image: centos:8
    command: ["/bin/sleep", "10000"]
    securityContext:
      privileged: true

2. edit the pod's network-selection-elements trying to define the name of the interface; i.e. turn the networks annotation into:

{
    "name": "macvlan1-config",
    "ips": [ "10.1.1.11/24" ],
    "interface": "net1"
}

Environment:

• multus-dynamic-networks-controller version: N/A
• Kubernetes version (use kubectl version): N/A
• Network-attachment-definition: N/A
• OS (e.g. from /etc/os-release): N/A
• Controller configuration (criSocketPath / multusSocketPath): N/A
• Kernel (e.g. uname -a): N/A
• Others: N/A

Additional info / context
I wonder if an event should be thrown for this vs only logging the error.