Detect valid SSH private keys that may require unlocking/decrypting correctly

Currently locked/encrypted SSH private keys are considered as invalid during client.SSHClient._auth(). An encrypted RSA key that I want to use during authentication (passed in as key_filename=/home/<user>/.ssh/id_rsa) is considered to be an invalid DSA(!) private key file.

The patch of this pull request fixes that.

Greets,
Mike