Fixed auto-increment install bug

Fixes http://www.concrete5.org/developers/bugs/5-6-2-1/install-fails-with-mysql-auto-increment-offset-set/

My initial thought was to detect group auto increment values that didn't match the defines in concrete/config/base.php and override those defines in config/site.php. Unfortunately this doesn't fix the install bug because site.php doesn't exist yet, and the temporary site_install.php doesn't get read until after base.php. Couldn't find a clean way to do this.

So instead I added an optional parameter to the Groups::add() function so the install routine can force the creation of groups with specific IDs instead of making assumptions about auto increment values. It defaults to null, which makes MySQL generate an auto increment value, so this doesn't change the API.