t3 instances not supported

[mojodna]

Summary

ecs-cli can't provision clusters using t3 instances:

ecs-cli up \
	  --keypair <redacted> \
	  --instance-role <redacted> \
	  --size 1 \
	  --instance-type t3.xlarge \
	  --cluster-config <redacted> \
	  --subnets <redacted> \
	  --vpc <redacted> \
	  --force \
	  --verbose
INFO[0001] Using recommended Amazon Linux AMI with ECS Agent 1.20.3 and Docker version 18.06.1-ce
INFO[0001] Created cluster                               cluster=<redacted> region=us-east-1
FATA[0002] Error executing 'up': ValidationError: Parameter EcsInstanceType failed to satisfy constraint: must be a valid EC2 instance type.

Like #64, the t3 types aren't included in https://github.com/aws/amazon-ecs-cli/blob/master/ecs-cli/modules/clients/aws/cloudformation/template.go#L56-L146

[BalmungSan]

Any plans to release this soon?

[allisaurus]

Support for t3 was added in 1.9.0. Resolving.