Add a config parameter to specify whether public IP is assigned

[pxsta]

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.

Vagrant.configure("2") do |config|
  config.vm.provider :aws do |aws,override|
    aws.subnet_id = SUBNET_ID
    aws.associate_public_ip = true
  end
end

[pxsta]

I'm sorry but I couldn't find an exact document of the fog.
This is the code for AssociatePublicIpAddress of the fog.
https://github.com/fog/fog/blob/v1.18.0/lib/fog/aws/parsers/compute/run_instances.rb#L77
https://github.com/fog/fog/blob/v1.18.0/lib/fog/aws/models/compute/server.rb#L15
https://github.com/fog/fog/blob/v1.18.0/lib/fog/aws/models/compute/server.rb#L178

[rtyler]

Thanks for the contribution @pxsta :)