Make flexible node allocations work with the --exclude flag

[vkarak]

The scenario is having a test with num_tasks = 0 and calling ReFrame with --reservation=foo --exclude=[nodelist].

[teojgo]

If num_tasks == 0 and a valid reservation is given, the actual num_tasks is set based on the reservation nodes satisfying the given partitions and constraints. Thus we must also check that a reservation node is not included in the nodelist given by the --exclude option. If it is, it must not count for the num_tasks that are calculated.