Add var to tests which enables stats checking

Add an additional variable FUEL_STATS_CHECK to
system tests settings which enables checking of
gathered usage statistics after performing tests
(currently only few tests support that). Existing
var FUEL_STATS_ENABLED is used for enabling stats
collecting in Nailgun settings.

Change-Id: I4250fd455b7149b2ef227e79b44170aef07f29ae
Implements: blueprint fuel-stats-test