Handle unlinking of SSH key instead of relying on tempfile

Perhaps it's a version/environment difference, but relying on tempfile to unlink the file when done seems to not be working on one host. I imagine it is because tempfile unlinks the handle when I called `f.close()`, so we're just going to handle unlinking ourselves and solve that problem permanently. This has the added benefit of us knowing that the file will be removed as soon as the transfer is complete. RSA keys on disk for a smaller amount of time "feels" better.