Change passed tests expectation

To get test_node.simple_cluster_flat change
expected result for passed test in OSTF invocation

Fixes bug/1247068

Change-Id: Ic96379442079479fbb5827ba95b0df5eac25e18a
Refactoring system tests on python-proboscis

Change-Id: Iae82bece4923db898836db405cddc81f94526913