Make `num_nodes` a test attribute and let scheduler backends decide how to treat it

[vkarak]

ReFrame historically has tried to avoid over-specification of the node request, so it officially supported only num_tasks and num_tasks_per_node as test attributes. This however can be problematic with some scheduler backends (see Slurm) where all three can be used at the same time. We should therefore make num_nodes a standard test attribute and let scheduler backends treat those options at their convenience.

I'm adding this initially to 5.0 as it might introduce breaking changes.