Fix and unify structure of modules in benchmark engine

*) Move benchmark.runner to benchmark.runners.base
*) Mova benchmark.base to benchmark.scenarios.base
*) Add context.base module that we will use in future
   for auto discovering, config validation & logging of
   context

This is the base step that is required before starting work around
task config refactoring & moving config validation to: context,
runner, and scenario base classes from benchmark.engine

bp benchmark-engine-refactoring

Change-Id: Icf9974947de34a9273cd2dc9f2f44b3ce563c8fe