IAllocator part 3: LUCreateInstance changes

This (final) patch allows the instance's nodes to be selected
automatically based on the passed allocator algorithm.

The patch changes the pnode opcode parameter from required to optional,
now either the pnode or the iallocator must be passed.

A possible improvement could be to organize all the _IAllocator
functions into a separate class, but that can come later and the current
version is functionally ok.

Reviewed-by: ultrotter