Update template with available instance types

[SoManyHs]

Reference:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-types.html#AvailableInstanceTypes

[SoManyHs]

Previous behavior:

~/go/src/github.com/aws/amazon-ecs-cli(master)$ ecs-cli up --instance-type f1.2xlarge --capability-iam --keypair my-keypair --force
INFO[0000] Created cluster                               cluster=test region=us-west-2
INFO[0000] Waiting for your CloudFormation stack resources to be deleted... 
INFO[0000] Cloudformation stack status                   stackStatus="DELETE_IN_PROGRESS"
INFO[0061] Cloudformation stack status                   stackStatus="DELETE_IN_PROGRESS"
INFO[0121] Cloudformation stack status                   stackStatus="DELETE_IN_PROGRESS"
FATA[0151] Error executing 'up': ValidationError: Parameter EcsInstanceType failed to satisfy constraint: must be a valid EC2 instance type.
	status code: 400, request id: ***-61f64caf175f 

With change:

~/go/src/github.com/aws/amazon-ecs-cli(update_instance_types)$ ecs-cli up --instance-type f1.2xlarge --capability-iam --keypair hhh-2
INFO[0000] Created cluster                               cluster=test region=us-west-2
INFO[0000] Waiting for your cluster resources to be created... 
INFO[0001] Cloudformation stack status                   stackStatus="CREATE_IN_PROGRESS"
INFO[0061] Cloudformation stack status                   stackStatus="CREATE_IN_PROGRESS"
INFO[0121] Cloudformation stack status                   stackStatus="CREATE_IN_PROGRESS"

~/go/src/github.com/aws/amazon-ecs-cli(update_instance_types)$ aws ecs list-container-instances --cluster test
{
    "containerInstanceArns": [
        "arn:aws:ecs:us-west-2:<my_account_id>:container-instance/********-****-****-****-c646981a7789"
    ]
}

~/go/src/github.com/aws/amazon-ecs-cli(update_instance_types)$ aws ec2 describe-instances --instance-ids i-****************

...
=>  "InstanceType": "f1.2xlarge",