lisa.wlgen.workload: rework
Fix long standing points of unsatisfaction:
- avoid API based on mutable
.output
attribute - Add a way to run in the background
- Move all the run() parameters into workload's constructor since some of them like
as_root
can impact the workload itself. (e.g. it's currently preventing the use of locked pages in rt-app since we don't know if we will be root or no)