[RestResourceHost] Fix the wrong count of bad hosts in the dashboard.

If the monitoring server doesn't have a host group at all, the server
is always counted as good in "Number of servers" on dashboard, although
"Number of hosts [with problem]" for the server is shown correctly.
Merge pull request #730 from project-hatohol/fix-show-wrong-bad-hosts-in-dashboard

[RestResourceHost] Fix the wrong count of bad hosts in the dashboard.