Update app_engine.feature_settings type and default

[aaron-lane]

modules/app_engine/variables.tf currently defines feature_settings as a generic list with a default of [{ enabled = true }]. The type should be updated to list(object{ split_health_checks = bool }) since feature_settings can only be defined once on google_app_engine_application and split_health_checks is the only valid argument. The default should be updated to { split_health_checks = true} since false enables legacy health checks.

https://www.terraform.io/docs/providers/google/r/app_engine_application.html#feature_settings

[bharathkkb]

@aaron-lane I can pick this up with #227

[aaron-lane]

@bharathkkb I incorrectly specified a type of list(object{ split_health_checks = bool }) when it should have been object{ split_health_checks = bool} as feature_settings can only be specified once. Can you make the necessary change?