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/config_test.go

@@ -1326,3 +1326,21 @@ func TestConfigMarketplaceDefaults(t *testing.T) {
 		require.Equal(t, "https://marketplace.example.com", *c.PluginSettings.MarketplaceUrl)
 	})
 }
+
+func TestSetDefaultFeatureFlagBehaviour(t *testing.T) {
+	cfg := Config{}
+	cfg.SetDefaults()
+
+	require.NotNil(t, cfg.FeatureFlags)
+	require.Equal(t, "off", cfg.FeatureFlags.TestFeature)
+
+	cfg = Config{
+		FeatureFlags: &FeatureFlags{
+			TestFeature: "somevalue",
+		},
+	}
+	cfg.SetDefaults()
+	require.NotNil(t, cfg.FeatureFlags)
+	require.Equal(t, "somevalue", cfg.FeatureFlags.TestFeature)
+
+}