Support for TF_KEY_NAME

[GeertJohan]

Hi,

I the TF_KEY_NAME override in the source, which is very useful because some servers are only accessable on their private_ip.
Should TF_KEY_NAME be documented? And will it stay supported?
Thanks

[adammck]

Yes, on both counts. It was added by #28, for the same use-case, but not documented at the time. It will continue to be supported indefinitely, although I'd like to add a --private argument to achieve the same thing.

[simt2]

It took me some time and debugging to find this hidden feature :)