scaleutils: remove setup burden on target plugin authors

[jrasell]

The current way in which the scaleutils are written puts a larger than required burden on the target plugin to parse and validate config params in order to populate a scale request object before calling scale util functions. Each horizontal cluster scaling target plugin will need to replicate this work, meaning it should be moved into the scaleutils so it happens in a single place.

This specifically relates to the creation of the ScaleInReq.