Add constants for commonly used values. (#3588)

This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.

This also fixes `goconst ./...` reported issues.

cmd/notify-webhook_test.go

@@ -40,7 +40,7 @@ func (p postHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 // Tests web hook initialization.
 func TestNewWebHookNotify(t *testing.T) {
-	root, err := newTestConfig("us-east-1")
+	root, err := newTestConfig(globalMinioDefaultRegion)
 	if err != nil {
 		t.Fatal(err)
 	}