WMI module update

Hi Team,

Due to the fact that I use massively the WMI windows services, I figured there's a problem on the poller when it tries to delete the removed services. This is due to a typo on 1 of the variables.

Attached there's a diff with: 

 - adjusted var name to be able to delete old services during the polling;
 - added an option to enable/disable the windows services polling per device and globally;
 - Improved the SQL queries for better performance; (this has reduced the WMI module polling time to 50%)

Please consider to review and patch.

Best regards,