Add a config parameter to specify whether public IP is assigned

I was implemented associate_public_ip option to assign a public IP address to an EC2 instance in a VPC.

This is for #205.

Please write Vagrantfile like this.
```bash
Vagrant.configure("2") do |config|
  config.vm.provider :aws do |aws,override|
    aws.subnet_id = SUBNET_ID
    aws.associate_public_ip = true
  end
end
```