Return the minion keys directory paths.

    This function is a copy of salt.key.Key._check_minions_directories.

def _check_minions_directories(pki_dir):
    '''
    Return the minion keys directory paths.

    This function is a copy of salt.key.Key._check_minions_directories.
    '''
    minions_accepted = os.path.join(pki_dir, salt.key.Key.ACC)
    minions_pre = os.path.join(pki_dir, salt.key.Key.PEND)
    minions_rejected = os.path.join(pki_dir, salt.key.Key.REJ)
    minions_denied = os.path.join(pki_dir, salt.key.Key.DEN)

    return minions_accepted, minions_pre, minions_rejected, minions_denied