Fix bug in ScenarioRunner

ScenarioRunner.run() method accidentally overwrites the 'clients' attribute of
the base scenario class with a dictionary. However, 'clients' should be
a method and so the overriding leads to an error during benchmarking.

Change-Id: I1551dbd177a03ff557ff135cf0cc1f49b63d1765
Closes-Bug: #1255039