Add support for Argo Rollouts

[iremmats]

My organization is using Argo Rollouts to allow us to do Canary and Blue Green deployments GitOps style. I would like to be able to use Keda in combination with this.

Use-Case

Allow Keda to create HPAs for other resources than just Deployments. Example of how the ScaledObject could look like.

scaleTargetRef:
    deploymentName: order-processor
    apiVersion: argoproj.io
    kind: Rollout

[zroubalik]

@iremmats thanks for the input. We are planning to implement a new KEDA version which should be able to scale more types dynamically: #591

[zroubalik]

merged to v2 branch