Set front_port=nil when network_mode is awsvpc
Set front_port=nil when network_mode is awsvpc

When networkMode=awsvpc, the host ports and container ports in port
mappings must match.
http://docs.aws.amazon.com/AmazonECS/latest/developerguide/launch_types.html#launch-type-fargate
In Hako, I decided to set front_port=nil to achieve the constraint while
scripts can be aware of such network configuration.

eagletmt/hako,eagletmt/hako,eagletmt/hako