Allow to configure empty ipam for macvlan

This PR add the option to configure an empty ipam for the macvlan cni plugin.

When using the macvlan cni plugin with an empty ipam the requeted pod will get the macvlan interface but without any ip address.

One of the use cases for this feature is for projects that runs a dhcp server inside the pod like KubeVirt.
In KubeVirt we need to let the vm running inside the pod to make the dhcp request so it will be able to make a release an renew request when needed.

plugins/main/macvlan/macvlan_test.go

@@ -286,6 +286,89 @@ var _ = Describe("macvlan Operations", func() {
 
 	})
 
+	It("configures and deconfigures a l2 macvlan link with ADD/DEL", func() {
+		const IFNAME = "macvl0"
+
+		conf := fmt.Sprintf(`{
+    "cniVersion": "0.3.1",
+    "name": "mynet",
+    "type": "macvlan",
+    "master": "%s",
+    "ipam": {}
+}`, MASTER_NAME)
+
+		targetNs, err := testutils.NewNS()
+		Expect(err).NotTo(HaveOccurred())
+		defer targetNs.Close()
+
+		args := &skel.CmdArgs{
+			ContainerID: "dummy",
+			Netns:       targetNs.Path(),
+			IfName:      IFNAME,
+			StdinData:   []byte(conf),
+		}
+
+		var result *current.Result
+		err = originalNS.Do(func(ns.NetNS) error {
+			defer GinkgoRecover()
+
+			r, _, err := testutils.CmdAddWithArgs(args, func() error {
+				return cmdAdd(args)
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			result, err = current.GetResult(r)
+			Expect(err).NotTo(HaveOccurred())
+
+			Expect(len(result.Interfaces)).To(Equal(1))
+			Expect(result.Interfaces[0].Name).To(Equal(IFNAME))
+			Expect(len(result.IPs)).To(Equal(0))
+			return nil
+		})
+		Expect(err).NotTo(HaveOccurred())
+
+		// Make sure macvlan link exists in the target namespace
+		err = targetNs.Do(func(ns.NetNS) error {
+			defer GinkgoRecover()
+
+			link, err := netlink.LinkByName(IFNAME)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(link.Attrs().Name).To(Equal(IFNAME))
+
+			hwaddr, err := net.ParseMAC(result.Interfaces[0].Mac)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(link.Attrs().HardwareAddr).To(Equal(hwaddr))
+
+			addrs, err := netlink.AddrList(link, syscall.AF_INET)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(len(addrs)).To(Equal(0))
+			return nil
+		})
+		Expect(err).NotTo(HaveOccurred())
+
+		err = originalNS.Do(func(ns.NetNS) error {
+			defer GinkgoRecover()
+
+			err := testutils.CmdDelWithArgs(args, func() error {
+				return cmdDel(args)
+			})
+			Expect(err).NotTo(HaveOccurred())
+			return nil
+		})
+		Expect(err).NotTo(HaveOccurred())
+
+		// Make sure macvlan link has been deleted
+		err = targetNs.Do(func(ns.NetNS) error {
+			defer GinkgoRecover()
+
+			link, err := netlink.LinkByName(IFNAME)
+			Expect(err).To(HaveOccurred())
+			Expect(link).To(BeNil())
+			return nil
+		})
+		Expect(err).NotTo(HaveOccurred())
+	})
+
 	It("configures and deconfigures a cniVersion 0.4.0 macvlan link with ADD/DEL", func() {
 		const IFNAME = "macvl0"