MM28858 Basic framework for use of feature flags. Enviroment variable overrides. (#15544)

* Basic framework for use of feature flags. Enviroment variable overrides.

* Use Apperr instead of error number increments.

* Undo random viper change.

* Update model/config_test.go

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

model/feature_flags.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+type FeatureFlags struct {
+	// Exists only for unit and manual testing.
+	// When set to a value, will be returned by the ping endpoint.
+	TestFeature string
+}
+
+func (f *FeatureFlags) SetDefaults() {
+	f.TestFeature = "off"
+}