<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def ssh_directory_for_unit(application_name, user=None):
 """Return the directory used to store ssh assets for the application. :param application_name: Name of application eg nova-compute-something :type application_name: str :param user: The user that the ssh asserts are for. :type user: str :returns: Fully qualified directory path. :rtype: str """

    if user:
        application_name = "{}_{}".format(application_name, user)
    _dir = os.path.join(NOVA_SSH_DIR, application_name)
    for d in [NOVA_SSH_DIR, _dir]:
        if not os.path.isdir(d):
            os.mkdir(d)
    for f in ['authorized_keys', 'known_hosts']:
        f = os.path.join(_dir, f)
        if not os.path.isfile(f):
            open(f, 'w').close()
    return _dir