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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def cluster_join(username, hostname):
    '''
    Join a Riak cluster

    .. versionchanged:: 2015.8.0

    CLI Example:

    .. code-block:: bash

        salt '*' riak.cluster_join <user> <host>

    username - The riak username to join the cluster
    hostname - The riak hostname you are connecting to
    '''
    ret = {'comment': '', 'success': False}

    cmd = __execute_cmd(
        'riak-admin', 'cluster join {0}@{1}'.format(username, hostname)
    )

    if cmd['retcode'] != 0:
        ret['comment'] = cmd['stdout']
    else:
        ret['comment'] = cmd['stdout']
        ret['success'] = True

    return ret