htools: generalise and rename availableLocalNodes

Currently this function works in the context of an instance, computing
its local group nodes. We can use generalise it to take a group index
argument, so that it's not bound to an instance, and we rename it
accordingly.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>