feat(convertNetconf): Add support for network config conversion

Adding the flag -convertNetconf which is used to specify the config
format to convert from (right now, only 'debian' is supported).
Once the network configs are generated, they are written to
systemd's runtime network directory and the network is restarted.

system/networkd.go

@@ -0,0 +1,89 @@
+package system
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net"
+	"os/exec"
+	"path"
+
+	"github.com/coreos/coreos-cloudinit/network"
+	"github.com/coreos/coreos-cloudinit/third_party/github.com/dotcloud/docker/pkg/netlink"
+)
+
+const (
+	runtimeNetworkPath = "/run/systemd/network"
+)
+
+func RestartNetwork(interfaces []network.InterfaceGenerator) (err error) {
+	defer func() {
+		if e := restartNetworkd(); e != nil {
+			err = e
+		}
+	}()
+
+	if err = downNetworkInterfaces(interfaces); err != nil {
+		return
+	}
+
+	if err = probe8012q(); err != nil {
+		return
+	}
+	return
+}
+
+func downNetworkInterfaces(interfaces []network.InterfaceGenerator) error {
+	sysInterfaceMap := make(map[string]*net.Interface)
+	if systemInterfaces, err := net.Interfaces(); err == nil {
+		for _, iface := range systemInterfaces {
+			sysInterfaceMap[iface.Name] = &iface
+		}
+	} else {
+		return err
+	}
+
+	for _, iface := range interfaces {
+		if systemInterface, ok := sysInterfaceMap[iface.Name()]; ok {
+			if err := netlink.NetworkLinkDown(systemInterface); err != nil {
+				fmt.Printf("Error while downing interface %q (%s). Continuing...\n", systemInterface.Name, err)
+			}
+		}
+	}
+
+	return nil
+}
+
+func probe8012q() error {
+	return exec.Command("modprobe", "8021q").Run()
+}
+
+func restartNetworkd() error {
+	_, err := RunUnitCommand("restart", "systemd-networkd.service")
+	return err
+}
+
+func WriteNetworkdConfigs(interfaces []network.InterfaceGenerator) error {
+	for _, iface := range interfaces {
+		filename := path.Join(runtimeNetworkPath, fmt.Sprintf("%s.netdev", iface.Name()))
+		if err := writeConfig(filename, iface.Netdev()); err != nil {
+			return err
+		}
+		filename = path.Join(runtimeNetworkPath, fmt.Sprintf("%s.link", iface.Name()))
+		if err := writeConfig(filename, iface.Link()); err != nil {
+			return err
+		}
+		filename = path.Join(runtimeNetworkPath, fmt.Sprintf("%s.network", iface.Name()))
+		if err := writeConfig(filename, iface.Network()); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func writeConfig(filename string, config string) error {
+	if config == "" {
+		return nil
+	}
+
+	return ioutil.WriteFile(filename, []byte(config), 0444)
+}