[engine-simplification] Simplify the input scheme for engine #1724

This issue is about simplifying the input scheme for the engine. With the introduction of oonirunv2.1, the input policies controlled by the engine can be easily replaced with the metadata that oonirun richer input provides us with. This allows us to kill redundant pieces of code and allow the client to control the settings of test inputs and environment.

The deliverables of this issue include:

• Reduce the current input policies in the engine to a single input processor
• Use the richer input metadata in oonirun to build experiment inputs