Support public subnets for clusters

[gregharvey]

You cannot currently opt for a public subnet for an ASG, our role automatically builds a private subnet. Such private subnets cannot work with ECS, for example, where the traffic comes in from NAT gateways and is effectively public, so you need a "publicly accessible" RDS instance on a public subnet for it to work. There are also cases where you might actually want your ASG to have public subnets, for example to save cost.

Key code is in this file:

• https://github.com/codeenigma/ce-provision/blob/2.x/roles/aws/aws_ec2_autoscale_cluster/tasks/subnet.yml