allow exclusion hostgroups

Hello, 

This change allows hostgroup verification for exclusion patterns as described in http://nagios.sourceforge.net/docs/3_0/objecttricks.html 

For instance,  the nagios_service: 
``` 
{
  "id": "load",
  "hostgroup_name": "base,!<valid-role>",
  "command_line": "$USER1$/check_nrpe -H $HOSTADDRESS$ -c check_load -t 20"
}
```
will now work as expected for a valid <valid-role> when compared to being dropped as a result of the hostgroup_name check.