generate ssl private key
    :param filepath: file path to the key file
    :param silent: whether to suppress output

def gen_private_key(filepath, silent=False):
    """
    generate ssl private key
    :param filepath: file path to the key file
    :param silent: whether to suppress output
    """
    cmd = 'openssl genrsa -out {} 2048'.format(filepath)
    message = 'generate private key {}'.format(filepath)
    call_openssl(cmd, message, silent)