Disable root_group on Windows

Fixes OHAI-490: http://tickets.opscode.com/browse/OHAI-490

Implementation of root_group for windows has been found to have poor
performance when the host is joined to an AD domain. Opting to disable
on Windows since large changes will be required for a performant
implementation.

Cherry-picked from master for 6-stable.

Conflicts:
	spec/unit/plugins/root_group_spec.rb
Merge branch 'OHAI-490-6-stable' into 6-stable