--ssh-key-filename doesn't support OpenSSH key format

[meeuw]

--ssh-key-filename doesn't support OpenSSH key format

It returns the following error message:

ValueError: q must be exactly 160, 224, or 256 bits long

q must be exactly 160, 224, or 256 bits long

As suggested in #772 this can be fixed by converting the SSH key to a supported format:

$ ssh-keygen -p -m PEM -f ~/.ssh/id_rsa.pem

mycli should give a better error message, possibly hint to the above conversion.