Move to upgraded gatling directory layout.

With the major upgrade to gatling here: https://github.com/apache/openwhisk/pull/4706
The layout for the simulations has changed, the build was adapted to use the old paths but we should move to the new layout to support future upstream changes.

What would the default setup look like? Should we move our stuff to conform to the default setup?

_Originally posted by @markusthoemmes in https://github.com/apache/openwhisk/pull/4706_