Add ENV TF_KEY_NAME to specify which IP to return

[anarcher]

I am not sure that this PR is useful. But I need it. :-)
For example, I want to use the terraform-inventory in a VPC.

$export TERRAFORM_INVENTORY_KEYNAME=private_ip
(go:terraform-inventory) anarch@booker ~/go/terraform-inventory/src/anarcher/terraform-inventory (master)
$./terraform-inventory --list
{ "instance" : ["10.90.0.2"],"name_instance" : ["10.90.0.35"] }

Anyway, I like the terraform-inventory. It's a good idea and very useful for me! :-)

Thanks,

[adammck]

This is a great idea, thanks! Would you mind changing the var name to something a little more concise? Maybe TF_KEY_NAME, to match the existing TF_STATE var?

[anarcher]

Yes, TF_KEY_NAME is better. I changed it. :-)