Setups the salt-ssh minion to be accessed with salt-ssh default key

def _add_ssh_key(ret):
    '''
    Setups the salt-ssh minion to be accessed with salt-ssh default key
    '''
    priv = None
    if __opts__.get('ssh_use_home_key') and os.path.isfile(os.path.expanduser('~/.ssh/id_rsa')):
        priv = os.path.expanduser('~/.ssh/id_rsa')
    else:
        priv = __opts__.get(
            'ssh_priv',
            os.path.abspath(os.path.join(
                __opts__['pki_dir'],
                'ssh',
                'salt-ssh.rsa'
            ))
        )
    if priv and os.path.isfile(priv):
        ret['priv'] = priv