Remove the key from the cache bank with all the key content.

def flush(bank, key=None):
    '''
    Remove the key from the cache bank with all the key content.
    '''
    _init_client()
    query = "DELETE FROM {0} WHERE bank='{1}'".format(_table_name, bank)
    if key is not None:
        query += " AND etcd_key='{0}'".format(key)

    cur, _ = run_query(client, query)
    cur.close()