terraform cloudformation template timeouts should be tweaked to match the CF ones

`aws_cloudformation_template` resources have two timeout notions. The first one is the one for CloudFormation. It is defined as a variable in our module and defaults to 60':
https://github.com/enthought/terraform-modules/blob/master/aws_docker_cluster/main.tf#L67

The second one is the Terraform timeout which defaults to 30' but is not defined in our module. See https://www.terraform.io/docs/providers/aws/r/cloudformation_stack.html#timeouts

We should at least synchronize the two.