Return the ssh_interface type to connect to. Either 'public_ips' (default)
    or 'private_ips'.

def ssh_interface(vm_):
    '''
    Return the ssh_interface type to connect to. Either 'public_ips' (default)
    or 'private_ips'.
    '''
    ret = config.get_cloud_config_value(
        'ssh_interface', vm_, __opts__, default='public_ips',
        search_global=False
    )
    if ret not in ('public_ips', 'private_ips'):
        log.warning(
            'Invalid ssh_interface: %s. '
            'Allowed options are ("public_ips", "private_ips"). '
            'Defaulting to "public_ips".', ret
        )
        ret = 'public_ips'
    return ret