Changes logic when no ssh_key_path has been set

[petems]

 IdentitiesOnly
             Specifies that ssh(1) should only use the authentication identity
             files configured in the ssh_config files, even if ssh-agent(1) or
             a PKCS11Provider offers more identities.  The argument to this
             keyword must be “yes” or “no”.  This option is intended for situ‐
             ations where ssh-agent offers many different identities.  The
             default is “no”.

So when we have an ssh that is not in a path (such as using an ssh key located on a smart card), you can still ssh to it! 👍

Also updates specs for this change

Closes #160

[rauchrob]

@petems Great, thank you!