API server explicitly notify systemd of successful startup

Use the systemd $NOTIFY_SOCKET convention for kube-apiserver
startup. This allows it to be part of dependency trees and for
consumers to wait until it is listening on its ports.

The $NOTIFY_SOCKET protocol is described here:

http://www.freedesktop.org/software/systemd/man/sd_notify.html

Currently this is limited to the kube-apiserver process. Other
kube processes are internal kubernetes moving points. The API
server is the entry point relied on by callers.

100% stolen from Stef Walter from:
https://github.com/GoogleCloudPlatform/kubernetes/pull/8316

Godeps/_workspace/src/github.com/coreos/go-systemd/daemon/sdnotify.go

@@ -0,0 +1,31 @@
+// Code forked from Docker project
+package daemon
+
+import (
+	"errors"
+	"net"
+	"os"
+)
+
+var SdNotifyNoSocket = errors.New("No socket")
+
+// SdNotify sends a message to the init daemon. It is common to ignore the error.
+func SdNotify(state string) error {
+	socketAddr := &net.UnixAddr{
+		Name: os.Getenv("NOTIFY_SOCKET"),
+		Net:  "unixgram",
+	}
+
+	if socketAddr.Name == "" {
+		return SdNotifyNoSocket
+	}
+
+	conn, err := net.DialUnix(socketAddr.Net, nil, socketAddr)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	_, err = conn.Write([]byte(state))
+	return err
+}