Add run_in_background attribute to scenarios

Add run_in_background attribute to scenarios

This change adds the possibility to run scenarios as "background
tasks".

Background scenarios/tasks:
  - are started before all "normal scenarios"
  - runs in parallel with "normal scenarios"
  - terminates when all "normal scenarios" have completed
    their tasks

They are intended as a way to perform background tasks, e.g. collect
data such as cpuload etc, in parallel with the execution of normal
benchmarking scenarios.

Note that we already have the 'run_in_parallel' attribute but
this attribute has a couple of issues and do not solve all the
uses cases.

Change-Id: I9c5230bfdbbb66030f57b658ce1db87ff2c2d62b
Signed-off-by: Jo¶rgen Karlsson <jorgen.w.karlsson@ericsson.com>