[DBTablesMonitoring] Revive host names of events

Host names are accidentaly removed by f9ca5c7aeb0ed935cd5babc894f1bdf701f0dab2
because default hostId in events table is 0, not INVLID_HOST_ID.

TODO: Should we fill it by INVALID_HOST_ID?
Merge pull request #556 from project-hatohol/revive-hostnames-of-events

[DBTablesMonitoring] Revive host names of events