mnp: Set hostname using downward API

MultusNetworkPolicy implementation default to `util.GetHostname`
to know the name of the node it is running on and filter pods accordingly.
In some cases, that information might be not accurate, leading to a
wrong behavior of the controller.

Use downward api to populate the `--hostname` argument of
the multus network policy controller.

Link: https://github.com/kubernetes/component-helpers/blob/v0.34.2/node/util/hostname.go#L28
Signed-off-by: Andrea Panattoni <apanatto@redhat.com>

Author: zeeke

bindata/network/multus-networkpolicy/multus-networkpolicy.yaml

@@ -41,6 +41,12 @@ spec:
         - "--network-plugins=macvlan,sriov,ipvlan,bond"
         - "--custom-v6-ingress-rule-file=/etc/multi-networkpolicy/rules/custom-v6-rules.txt"
         - "--custom-v6-egress-rule-file=/etc/multi-networkpolicy/rules/custom-v6-rules.txt"
+        - "--hostname=${K8S_NODE_NAME}"
+        env:
+        - name: K8S_NODE_NAME
+          valueFrom:
+            fieldRef:
+              fieldPath: spec.nodeName
         resources:
           requests:
             cpu: "100m"