etcdctlv3: handle empty options related to security

Current etcdctlv3 doesn't handle options related to security when they
are not passed. Connections cannot be established and etcd produces
logs like this:

```
14:09:07 etcd1 | 2016/02/04 14:09:07 transport: http2Server.HandleStreams received bogus greeting from client: "\x16\x03\x01\x00\x9a\x01\x00\x00\x96\x03\x03\xf6\t\xda\x06QV\xb4\xdd\xc1gF\x1cC"
```

This commit fixes the problem. In addition, a case that empty strings
are passed to the options (e.g. --key="") are treated as error.

etcdctlv3/command/global.go

@@ -15,6 +15,7 @@
 package command
 
 import (
+	"errors"
 	"time"
 
 	"github.com/coreos/etcd/Godeps/_workspace/src/github.com/spf13/cobra"
@@ -38,15 +39,28 @@ func mustClient(cmd *cobra.Command) *clientv3.Client {
 	// set tls if any one tls option set
 	var cfgtls *transport.TLSInfo
 	tls := transport.TLSInfo{}
-	if tls.CertFile, err = cmd.Flags().GetString("cert"); err == nil {
+	var file string
+	if file, err = cmd.Flags().GetString("cert"); err == nil && file != "" {
+		tls.CertFile = file
 		cfgtls = &tls
+	} else if cmd.Flags().Changed("cert") {
+		ExitWithError(ExitBadArgs, errors.New("empty string is passed to --cert option"))
 	}
-	if tls.KeyFile, err = cmd.Flags().GetString("key"); err == nil {
+
+	if file, err = cmd.Flags().GetString("key"); err == nil && file != "" {
+		tls.KeyFile = file
 		cfgtls = &tls
+	} else if cmd.Flags().Changed("key") {
+		ExitWithError(ExitBadArgs, errors.New("empty string is passed to --key option"))
 	}
-	if tls.CAFile, err = cmd.Flags().GetString("cacert"); err == nil {
+
+	if file, err = cmd.Flags().GetString("cacert"); err == nil && file != "" {
+		tls.CAFile = file
 		cfgtls = &tls
+	} else if cmd.Flags().Changed("cacert") {
+		ExitWithError(ExitBadArgs, errors.New("empty string is passed to --cacert option"))
 	}
+
 	cfg := clientv3.Config{
 		Endpoints:   []string{endpoint},
 		TLS:         cfgtls,