Add option for no feature management section found

[zhiyuanliang-ms]

Resolve #308

[jimmyca15]

I don't see a need for FeatureManagementOptions to be updated. In fact it's dangerous. If IFeatureDefinitionProvider that is created with the inline configuration is not the one that ends up being used (multiple addfeaturemanagmeent calls), then the options won't match the added service.

I think it needs to be

            services.TryAddSingleton<IFeatureDefinitionProvider>(sp =>
                new ConfigurationFeatureDefinitionProvider(
                    configuration, 
                    sp.GetRequiredService<ILoggerFactory>())
                {
                    UseTopLevelConfiguration = true
                });