Get connection to haproxy socket.

def _get_conn(socket=DEFAULT_SOCKET_URL):
    '''
    Get connection to haproxy socket.
    '''
    assert os.path.exists(socket), '{0} does not exist.'.format(socket)
    issock = os.stat(socket).st_mode
    assert stat.S_ISSOCK(issock), '{0} is not a socket.'.format(socket)
    ha_conn = haproxy.conn.HaPConn(socket)
    return ha_conn