🌱 Set MachineDeployment selector in managed clusters

[fabriziopandini]

What this PR does / why we need it:
As of today when we are creating MachineDeployments in managed cluster we are not setting spec.selector field, and thus it gets initialised by web hooks to something like

  selector:
    matchLabels:
      cluster.x-k8s.io/cluster-name: my-cluster1
      cluster.x-k8s.io/deployment-name: my-cluster1-md1-v8hsl

However, given that selector labels are enforced in spec.template.metadata where we are already adding topology specific labels, so we end up with

  template:
    metadata:
      labels:
        cluster.x-k8s.io/cluster-name: my-cluster1
        cluster.x-k8s.io/deployment-name: my-cluster1-md1-v8hsl
        topology.cluster.x-k8s.io/deployment-name: md1
        topology.cluster.x-k8s.io/owned: ""

And the co-existance of both cluster.x-k8s.io/deployment-name and topology.cluster.x-k8s.io/deployment-name might be confusing at first sight (both at MD level and also on MS and Machines down the chain).

This PR is proposing a change in how we generate the generated cluster so the resulting machine deployment object will be:

 selector:
    matchLabels:
      cluster.x-k8s.io/cluster-name: my-cluster1
      topology.cluster.x-k8s.io/deployment-name: md1
      topology.cluster.x-k8s.io/owned: ""
  template:
    metadata:
      labels:
        cluster.x-k8s.io/cluster-name: my-cluster1
        topology.cluster.x-k8s.io/deployment-name: md1
        topology.cluster.x-k8s.io/owned: ""

With a simpler and cleaner label set (both at MD level and also on MS and Machines down the chain).

[vincepri]

/approve
/lgtm

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