Make the config loading testable

This is needed to verify  #2309, but it closes a big hole in Hugo's automated tests.

The loading of the config is now moved to `hugolib` and the same default settings are now used
in production and tests.

As Viper now uses Afero as its filesystem, we now can write fairly complete integration tests with ease.

See #2309

hugolib/hugo_sites_test.go

@@ -29,18 +29,8 @@ func init() {
 func testCommonResetState() {
 	hugofs.InitMemFs()
 	viper.Reset()
-	viper.Set("ContentDir", "content")
-	viper.Set("DataDir", "data")
-	viper.Set("I18nDir", "i18n")
-	viper.Set("themesDir", "themes")
-	viper.Set("LayoutDir", "layouts")
-	viper.Set("PublishDir", "public")
-	viper.Set("RSSUri", "rss")
-
-	viper.Set("Taxonomies", map[string]interface{}{
-		"tag":      "tags",
-		"category": "categories",
-	})
+	viper.SetFs(hugofs.Source())
+	loadDefaultSettings()
 
 	if err := hugofs.Source().Mkdir("content", 0755); err != nil {
 		panic("Content folder creation failed.")