use ssh paths from tugboat file

my ssh key is ~/.ssh/id_rsa and is named 'trimix' on digital ocean.

trying to find ~/.ssh/trimix[.pem] won't work for me.

could be generalized more, but i'm not super familiar with this code...
Can you also check that it's not empty string?  When you set up a tugboat file and don't specify a thing it becomes empty.