Migrate to using aws_launch_template vs. aws_launch_configuration

[hectcastro]

Looks like this is the new way to provide instance level parameters for ASGs. After a quick review, it appears that all of the arguments we're currently providing to aws_launch_configuration are supported by aws_launch_template (block_device_mappings looks like the only one that isn't straightforward).

The motivation for migrating to aws_launch_template is to support enabling T2 Unlimited support via credit_specification.

See: https://www.terraform.io/docs/providers/aws/r/launch_template.html

[hectcastro]

Unblocked by hashicorp/terraform-provider-aws#5632, which is in v1.34.0+ of the AWS Terraform provider.

[rbreslow]

• Merge Migrate to using aws_launch_template vs. aws_launch_configuration and add support for T2 Unlimited #34
• Merge Clarify ECS cluster security group usage #37
• Cut a release on terraform-aws-ecs-cluster

I feel like I'm missing something. I believe there was another Terraform module we wanted to cut a release on in tandem with merging this change.

Also, with #34, I believe we paired and setup testing environments out-of-band, which is why the testing instructions were left unfinished. I am going to test again and I will update the PR body.

[hectcastro]

I don't think that we have to bundle the issues referenced with this one.

I think that the two modules we wanted to release in tandem was this and terraform-aws-ecs-web-service. That was for the autoscaling role ARN removal, but we already did that.

I updated the status of this issue, but it isn't in our current sprint yet, so I would hold off on updating the PR until we add it to a sprint.