[hatohol-arm-plugin-zabbix] Fix a bug that doesn't update ArmInfo.

As a result, the correct status is not shown in the Server page on WebUI.
Merge pull request #291 from project-hatohol/fix-bug-status-of-hap-zabbix-api-not-shown

[hatohol-arm-plugin-zabbix] Fix a bug that doesn't update ArmInfo.