browser-flag: wrapped bool type denotes browser on/off flag. (#3963)

Statically typed BrowserFlag prevents any arbitrary string value
usage. The wrapped bool marshals/unmarshals JSON according to the
typed value ie string value "on" represents boolean true and "off" as
boolean false.

cmd/config-v16_test.go

@@ -159,7 +159,7 @@ func TestServerConfigWithEnvs(t *testing.T) {
 	defer removeAll(rootPath)
 
 	// Check if serverConfig has
-	if serverConfig.GetBrowser() != "off" {
+	if serverConfig.GetBrowser() {
 		t.Errorf("Expecting browser `off` found %s", serverConfig.GetBrowser())
 	}