[Forecast Feature] - Validate ECS EC2 instances have access to the ECS service

[kddejong]

When launching ECS EC2 instances I want to validate the subnets have a path to the ECS service to talk with the ECS service.

This can be done by access to NAT gateway, Internet Gateway, or endpoints. ASGs will happily launch these instances but they will never register as available capacity for the ECS cluster.

This can also validate that the route table for the subnet being used also doesn't have any black holes (which can be a validation item for anything launching into a subnet).