[server][HostResourceQueryOption] Use more readable name

`HostResourceQueryOption::Synapse::hostgroupIdColumnIdx` was used for
determining whether the option needs to join with hostgroup table.

If `hostgroupIdColumnIdx` isn't `INVALID_COLUMN_IDX`, it doesn't need to
join with hostgroup table.

`hostgroupIdColumnIdx` wasn't straightforward name. So this change uses
straightforward name.

See also https://github.com/project-hatohol/hatohol/commit/e2fbebfc2b41fa48d5582a1c69529092be3d1768#commitcomment-7440124