[compute image builder] [AWS} add flag for public IP association
Add a flag that can be used to disable the association of a public IP address for the node that Packer uses to build a new images, when building on AWS. A public IP is not needed when that node is reachable via the internal IP, and in some environments public IPs are not allowed by policy.
The option should default to "true": in the common case a public IP is desired.
Likewise, add a flag for ENA support. In some environments this feature is not available. Default it to "true".