(PUP-266) Ensure ADSI Group SIDs may be looked up

- Previously, calling Puppet::Util::ADSI::Group.exists? for a
well-known group SID would return false, because the WinNT:// style
URI used to find the group would not examine / lookup the SID, but
would instead create a broken Uri that the underlying OS could not
understand
- Now, the given group name is checked to see if it's a SID before the
Uri is constructed, and a SID style Uri is created where appropriate