Avoid ListBuckets() call instead rely on simple HTTP GET (#8475)

This is to avoid making calls to backend and requiring
gateways to allow permissions for ListBuckets() operation
just for Liveness checks, we can avoid this and make
our liveness checks to be more performant.

cmd/gateway/azure/gateway-azure.go

@@ -185,7 +185,9 @@ func (g *Azure) NewGatewayLayer(creds auth.Credentials) (minio.ObjectLayer, erro
 	c.HTTPClient = &http.Client{Transport: minio.NewCustomHTTPTransport()}
 
 	return &azureObjects{
-		client: c.GetBlobService(),
+		endpoint:   fmt.Sprintf("https://%s.blob.core.windows.net", creds.AccessKey),
+		httpClient: c.HTTPClient,
+		client:     c.GetBlobService(),
 	}, nil
 }
 
@@ -343,7 +345,9 @@ func azurePropertiesToS3Meta(meta storage.BlobMetadata, props storage.BlobProper
 // azureObjects - Implements Object layer for Azure blob storage.
 type azureObjects struct {
 	minio.GatewayUnsupported
-	client storage.BlobStorageClient // Azure sdk client
+	endpoint   string
+	httpClient *http.Client
+	client     storage.BlobStorageClient // Azure sdk client
 }
 
 // Convert azure errors to minio object layer errors.
@@ -447,6 +451,8 @@ func (a *azureObjects) Shutdown(ctx context.Context) error {
 
 // StorageInfo - Not relevant to Azure backend.
 func (a *azureObjects) StorageInfo(ctx context.Context) (si minio.StorageInfo) {
+	si.Backend.Type = minio.BackendGateway
+	si.Backend.GatewayOnline = minio.IsBackendOnline(ctx, a.httpClient, a.endpoint)
 	return si
 }