[#4314] Inject ControllerManager in Forward plugin

- This was the old way of getting a controller to forward to:
  - Grab the currently set controller
  - Grab the service locator from the controller
  - Grab the ControllerLoader service from the service locator
  - Grab the new controller from the ControllerLoader service
- New way:
  - ForwardFactory injects the ControllerManager instance into the
    Forward instance on creation.
  - Forward plugin pulls the new controller from the composed
    ControllerManager.

Better separation of concerns, better usage of Dependency Injection,
controllers no longer need to be ServiceManagerAware in order to forward
(they just need the PluginManager injected).
[#4330] Constructor injection

- Use constructor injection for getting ControllerManager into Forward plugin
  (per @bakura10)
- Remove test case that's no longer applicable (we can now forward to any
  controller!)
- Fix test cases that were instantiating the Forward plugin directly to inject
  the ControllerManager
Incorporate feedback

- Since we're doing constructor injection, the setter is not necessary,
  and neither is the getter. As the setter did not exist previously, and
  the getter was protected previously, they can safely be removed.
- Renamed $locator to $controllers to make the name more semantic.
Merge branch 'feature/4314' of github.com:weierophinney/zf2 into weierophinney-feature/4314
Merge branch 'weierophinney-feature/4314' into develop
Close #4330