Add an AWS packer template

[rbeede]

Issue Description

Add an AWS packer template so a build can be created to deliver an AMI.

This was a fork that implemented this, but the fork is behind the commits and thus broken. It also only covered Ubuntu 14.04 and not Windows.

https://github.com/jocic/AWS.Metasploitable3/blob/master/packer/templates/ubuntu_1404.json

This was raised once before, but the issue was closed with no resolution.

#76

[rbeede]

https://github.com/rapid7/metasploitable3/blob/master/packer/templates/aws/ubuntu_1404_ctf_2017.json

Perhaps this but remove
"region": "us-east-1",

and instruct the user to

export AWS_DEFAULT_REGION=us-east-1

So it can be specified by running

packer build packer/templates/aws/ubuntu_1404.json

[rbeede]

Also add a Windows AWS template for packer

[rbeede]

Also blocked by #441