Add "Scenario" class wait methods for runners

Add "Scenario" class wait methods for runners

"Duration" and "Iteration" runners execute a passive wait during the
execution of the work process. This wait time is done at the end of
the scenario "run" method execution.

This patch adds a pre-run and post-run wait period, which will
depends on the Scenario executed. The wait time will be always the
same, but the execution order (pre-wait time, run method, post-wait
time) will depends on the Scenario.

By default, any Scenario will execute the "run" method and them will
wait the specified time.

NetworkServicesTestCase Scenario will wait the specified time and them
will execute the "run" method to retrieve the KPIs.

JIRA: YARDSTICK-1067

Change-Id: I6ad6bfc6978815b6b2d4df63f2ac2f8815fb5b8a
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>