[HapProcessZabbixAPI] Fix a bug that fetching items doesn't work

In the previous implementation fetchItem() doesn't work and it
causes stall with following debug message:

[DBG] <HatoholException.cc:48> HatoholException: <HatoholArmPluginInterface.cc:298> ASSERTION failed: [m_impl->currMessage] : This object doesn't have a current message.

The cause is that it doesn't use MessagingContext passed via
argument.
Merge pull request #722 from project-hatohol/fix-fetch-item-of-hap-zabbix

[HapProcessZabbixAPI] Fix a bug that fetching items doesn't work