Revert vendor directory to support old golang (GOPATH mode)

gomodule is still in progress to migrate for now, hence multus
team decide to keep vendor directory to support build without
gomodule.

vendor/github.com/modern-go/concurrent/log.go

@@ -0,0 +1,13 @@
+package concurrent
+
+import (
+	"os"
+	"log"
+	"io/ioutil"
+)
+
+// ErrorLogger is used to print out error, can be set to writer other than stderr
+var ErrorLogger = log.New(os.Stderr, "", 0)
+
+// InfoLogger is used to print informational message, default to off
+var InfoLogger = log.New(ioutil.Discard, "", 0)