FR: Add num_nodes/nodes to the performance log

[mredenti]

When using the ReFrame configuration setting:

sched_options:
  use_nodes_option: True

it would be useful to have ReFrame log the number of allocated nodes. At the moment there is a %(check_job_nodelist)s log property but an additional %(check_(num_)nodes)s would be more convenient for streamlining job result parsing and reporting.

[vkarak]

We are thinking of making num_nodes a standard test attribute in #3491 in which case this issue will be resolved automatically.