Move default directory for exec plugin into exec plugin

pkg/kubelet/network/exec/exec.go

@@ -85,11 +85,16 @@ const (
 	setUpCmd    = "setup"
 	tearDownCmd = "teardown"
 	statusCmd   = "status"
+	defaultDir  = "/usr/libexec/kubernetes/kubelet-plugins/net/exec/"
 )
 
 func ProbeNetworkPlugins(pluginDir string) []network.NetworkPlugin {
 	execPlugins := []network.NetworkPlugin{}
 
+	if pluginDir == "" {
+		pluginDir = defaultDir
+	}
+
 	files, _ := ioutil.ReadDir(pluginDir)
 	for _, f := range files {
 		// only directories are counted as plugins