(PUP-1559) 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
  
  Paired-with: Joshua Partlow <jpartlow@puppetlabs.com>