Avoid ListenBucket targets to be listed in ServerInfo (#6340)

In current master when you do `mc watch` you can see a
dynamic ARN being listed which exposes the remote IP as well

```
mc watch play/airlines
```

On another terminal
```
mc admin info play
●  play.minio.io:9000
   Uptime : online since 11 hours ago
  Version : 2018-08-22T07:50:45Z
   Region :
 SQS ARNs : arn:minio:sqs::httpclient+51c39c3f-131d-42d9-b212-c5eb1450b9ee+73.222.245.195:33408
    Stats : Incoming 30GiB, Outgoing 7.6GiB
  Storage : Used 7.7GiB
```

SQS ARNs listed as part of ServerInfo should be only external targets,
since listing an ARN here is not useful and it cannot be re-purposed in
any manner.

This PR fixes this issue by filtering out httpclient from the ARN list.

This is a regression introduced in #5294 0e4431725c

cmd/notification.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"net/url"
 	"path"
+	"strings"
 	"sync"
 	"time"
 
@@ -46,7 +47,13 @@ func (sys *NotificationSys) GetARNList() []string {
 	arns := []string{}
 	region := globalServerConfig.GetRegion()
 	for _, targetID := range sys.targetList.List() {
-		arns = append(arns, targetID.ToARN(region).String())
+		// httpclient target is part of ListenBucketNotification
+		// which doesn't need to be listed as part of the ARN list
+		// This list is only meant for external targets, filter
+		// this out pro-actively.
+		if !strings.HasPrefix(targetID.ID, "httpclient+") {
+			arns = append(arns, targetID.ToARN(region).String())
+		}
 	}
 
 	return arns