Clone cmdlib._GetWantedNodes into _GetWantedInstances

This duplicates _GetWantedNodes to _GetWantedInstances, after doing some
changes to it:
  - fix an indentation error that should result in only the last node
    name passed being chosen
  - change the function to have a single return statement
  - remove a redundant 'is not None' check

Reviewed-by: imsnah