Support for flexible node allocation per virtual partition

[vkarak]

The idea is that we need to extend the feature that we already have for the flexible node allocation inside a reservation, to whole virtual partitions of the system. Moreover, the following combination need to be supported:

• If --nodelist is passed, the test should be launched on the selected nodes.
• If --exclude-nodes is passed, the test should be launched on all the nodes of the virtual partition except the selected ones.
• Respect maximum job size (e.g., 2400 nodes)
• By default, submit to the available nodes. If --nodelist is passed, then these nodes must be used.
• What if both --nodelist and --exclude-nodes are passed and the sets intersect?

From the design's point of view, this functionality (for reservations) is part of the Slurm backend. I think it would be good if we extend the Job API so as the backends automatically figure out the number of tasks for the test. Again, implementations for this feature, we will only provide for the Slurm backend.