These changes permit the use of pre-created SSH keypairs with AWS. If
so, the configuration for the builder needs to include an
ssh_keypair_name option and a ssh_private_key_file.

If ssh_private_key_file is *not* defined, it'll go through the
rigamarole of creating a temporary keypair. The ssh_keypair_name option
by itself won't make that change, because it doesn't make sense to
specify a keypair but not tell packer where the private key is, but it
does happen that you could have a private key and the public-key is
"baked in", and not part of your EC2 account.