Add UTCNow() function. (#3931)

This patch adds UTCNow() function which returns current UTC time.

This is equivalent of UTCNow() == time.Now().UTC()

cmd/xl-v1-metadata_test.go

@@ -1,5 +1,5 @@
 /*
- * Minio Cloud Storage, (C) 2015, 2016 Minio, Inc.
+ * Minio Cloud Storage, (C) 2015, 2016, 2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -169,7 +169,7 @@ func isXLMetaSimilar(m, n xlMetaV1) bool {
 func TestPickValidXLMeta(t *testing.T) {
 	obj := "object"
 	x1 := newXLMetaV1(obj, 4, 4)
-	now := time.Now().UTC()
+	now := UTCNow()
 	x1.Stat.ModTime = now
 	invalidX1 := x1
 	invalidX1.Version = "invalid-version"