new variables 'associate_public_ip_address' and 'target_ami_name_prefix'

[charlestjackson]

Description

Two new variables added to Packer config.

Motivation and Context

Fixes issue 64

How Has This Been Tested?

These changes were tested by running against an AWS account using a non-default VPC and a public subnet that was not configured to auto-assign public IPs.

Screenshots (if appropriate):

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