lisa.wlgen.rta: Strenghten sanity check on calibration values

* Check against the fastest CPU in each capacity class. This CPU is the
  one that has been the less impacted by external disturbances, that are
  usually random.

* Check that the rt-app calibration values are inversely proportional to
  the CPU capacities. When the scaling factor diverges too much for some
  CPUs, raise an exception. This indicates that rt-app will be unable to
  provide predictable utilizations across different CPUs, which is the
  underlying assumption of all wlgen-based code in LISA.

* Use the CPU capacities from plat_info rather than trying to fetch them
  directly using devlib. This follows the practice in LISA, allowing the
  user to provide the values in a configuration file in order to run as
  non-root (it's also faster and uses values printed in the log, for a
  consistent debugging experience).