virt: Cartesian config update v2. RFC

***Pull-request is based on cart-bug-speed. After pulling this pull-request there will only stay 6 commit of 8.

Features:
<ul>
     <li> virt: Adds possibility to exclude filter from config. </li>
     <li> virt: Adds possibility filter defaults variant from variants </li>
     <li> virt: Adds named variants to Cartesian config.  </li>
</ul>
Unittest:
<ul>
     <li> virt: Extends Cartesian config unittests for new features     </li>
        <ul>
            <li> and replace exec call by better version</li>
        </ul>
</ul>Debug:
<ul>
     <li> virt: Make Cartesian config more verbose </li>
     <li> virt: Adds formal definition for Cartesian config </li>
</ul>