Initialize users database

    initialize database and create necessary tables
    to handle users oprations.

    :param dbURL: database url, as described in :func:`init_proxy`

def init_db(dbURL, pwd_salt_size=None, pwd_rounds=None):
    '''Initialize users database

    initialize database and create necessary tables
    to handle users oprations.

    :param dbURL: database url, as described in :func:`init_proxy`
    '''
    if not dbURL:
        dbURL = 'sqlite:///:memory:'
    logging.getLogger(__name__).debug("Initializing database: {}".format(dict(url=dbURL,
                                                                              pwd_salt_size=pwd_salt_size,
                                                                              pwd_rounds=pwd_rounds)))
    try:
        db = init_proxy(dbURL)
        global pwdCryptCtx
        pwdCryptCtx = gen_crypt_context(salt_size=pwd_salt_size, rounds=pwd_rounds)
        create_tables(db)
        return db
    except Exception as e:
        e.args = (e.args[0] + ' [users database]',)
        raise