Refactor controlplane controllers to eliminate code duplication

[stoyanr]

What this PR does / why we need it:
Refactors the AWS and GCP controlplane actuators into a generic actuator and provider-specific "value providers" in order to eliminate code duplication.

Special notes for your reviewer:

• The chosen approach has been agreed with @rfranzke. It allows for significantly reducing the code duplication between the current controlplane controllers, while at the same time keeping the flexibility to provide a very different implementation in the future if needed.
• The generic actuator is just one possible implementation of the Actuator interface. Providers might choose to use the generic actuator and plug into it a ValuesProvider implementation, or they might choose to provide their own Actuator if they need to do something different from the flow that the generic actuator implements.
• Charts and image vectors are still provider-specific, they are very similar right now but we expect they might diverge quickly.
• Tests for the GCP ValuesProvider have been added.
• Depends on Add GCP controlplane controller #69 which should be merged first.

Release note:

Refactor controlplane controllers to eliminate code duplication

[rfranzke]

/lgtm