kubeadm implement preflight checks

Includes checks for verifying services exist and are enabled, ports are
open, directories do not exist or are empty, and required binaries are
in the path.

Checks that user running kubeamd init and join is root and will only execute
command if user is root. Moved away from using kubectl error handling to
having kubeadm handle its own errors. This should allow kubeadm to have
more meaningful errors, exit codes, and logging for specific kubeadm use
cases.

cmd/kubeadm/kubeadm.go

@@ -21,12 +21,12 @@ import (
 	"os"
 
 	"k8s.io/kubernetes/cmd/kubeadm/app"
+	"k8s.io/kubernetes/cmd/kubeadm/app/util"
 )
 
-// TODO(phase1+): check for root
 func main() {
 	if err := app.Run(); err != nil {
-		fmt.Printf(app.AlphaWarningOnExit)
+		fmt.Printf(util.AlphaWarningOnExit)
 		os.Exit(1)
 	}
 	os.Exit(0)