Suppress logging before flag.Parse from glog (#2970)

Replace `goflag.Parse()` call with `goflag.CommandLine.Parse([]string{})`
This serves multiple purposes
1. We don't actually parse the os.Args (which Parse() method internally does). This ensures that the command line arguments are parsed only by RootCmd.Execute() call (which is the parent command)
2. We suppress the `Error: Logging before flag.Parse...` warning messages. https://github.com/golang/glog/blob/master/glog.go#L679 causes the warning message and we suppress this warning by calling the Parse method on the underlying flagset.

Signed-off-by: Ibrahim Jarif <[email protected]>

dgraph/cmd/root.go

@@ -18,8 +18,6 @@ package cmd
 
 import (
 	goflag "flag"
-	"fmt"
-	"os"
 	"strings"
 
 	"github.com/dgraph-io/dgraph/dgraph/cmd/alpha"
@@ -56,11 +54,12 @@ cluster.
 // This is called by main.main(). It only needs to happen once to the rootCmd.
 func Execute() {
 	initCmds()
-	goflag.Parse()
-	if err := RootCmd.Execute(); err != nil {
-		fmt.Println(err)
-		os.Exit(1)
-	}
+
+	// Convinces goflags that Parse() has been called to avoid noisy logs.
+	// https://github.com/kubernetes/kubernetes/issues/17162#issuecomment-225596212
+	x.Check(goflag.CommandLine.Parse([]string{}))
+
+	x.Check(RootCmd.Execute())
 }
 
 var rootConf = viper.New()