Use --switches=<count> and max_switch_wait

[laraPPr]

This should be implemented similarly to the the memory hook.

[casparvl]

Proposed approach:

• Add an 'extras' field in the config where system owners can declare how many nodes they have on a single leaf swtich
• Add a 'resources' field in the config to request all nodes from a single switch
• Add to eessi_mixin: logic to check how many nodes there are to a switch, and if the requested node count is lower than that, ask for a single switch.

[casparvl]

Good to note: @satishskamath experimented and noticed that if you ask e.g. 200 nodes on a single switch, but your system has 16 nodes per switch, the job will not be rejected (which is good), but you will wait for the max_switch_wait time (which is bad). That's why we proposed the above approach and force the end-user to configure this in the ReFrame config.