Added the option to define AD domain manually

Hi Jiji,

I've been trying to run Sonar with this plugin on a linux box, which name is different than the AD domain I want to integrate Sonar with. That means auto discovery does not work for me, so I've extended the plugin to allow manual override of the domain name in `sonar.properties` file using `sonar.ad.domain` setting.

Cheers,
Sergey