Modify the list_hosts to return the online hosts

[knwng]

When I use nebula2-python to connect to a nebula cluster deployed by docker-compose, the MetaCache will return an invalid storage address(0.0.0.0:9779) following some valid addresses. But currently the invalid address will crash the entire program.

WARNING:root:storage_addrs: [HostAddr(
    host='0.0.0.0',
    port=9779), HostAddr(
    host='storaged0',
    port=9779), HostAddr(
    host='storaged1',
    port=9779), HostAddr(
    host='storaged2',
    port=9779)]
ERROR:root:Create storage connection to HostAddr(
    host='0.0.0.0',
    port=9779) failed: socket error connecting to host 0.0.0.0, port 9779 (('0.0.0.0', 9779)): ConnectionRefusedError(111, 'Connection refused')

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[laura-ding]

Thanks for your contribution!