OHAI-491: Implement root_group for Windows 

Due to Windows-only bug [OHAI-490](https://tickets.opscode.com/browse/OHAI-490), the root_group plug-in was disabled for Windows. This change restores the functionality, and uses a purely local SID translation operation to avoid the issue we hit in the earlier implementation based on an inefficient WMI provider for the Win32_Group class. The LookupAccountSid method used in this implementation does a purely local lookup on one particular well-known SID, the one for the administrators group, and translates it to a name.

So the number of groups defined by the local system is not relevant, and certainly groups defined in AD will have no impact on the performance -- it's a simple local lookup of one SID.
Yeah, but I think the likelihood of slowdown for our CI at least is negligible. Current tests run on non-domain-joined systems by default, so they won't hit the issue that required this fix in the first place. And even on domains, a test domain like ours will not have 100K groups and connectivity will be good (if it isn't, specs will fail before they ever hit this test).

So for now at least I'm ok with the use of WMI here since we should not hit the slow path. Plus, the Windows team might decide to rename this group out of the box some day... stranger things have occurred.