Add the node's UID to the machine's noderef.

[spew]

What this PR does / why we need it:
This PR adds the node's UID to the machine's noderef field. This will allow the machine controller to definitively know what node belongs to a machine.

To test I ran a custom machine-controller, after doing so, the ouput of kubectl get machines:

$ kubectl get machines -o json | jq '.items[].status'
{
  "lastUpdated": "2018-06-14T23:02:09Z",
  "nodeRef": {
    "kind": "Node",
    "name": "gce-master-qqn5q",
    "uid": "18afd236-7023-11e8-b23f-42010a8a0002"
  },
  "providerStatus": null
}
{
  "lastUpdated": "2018-06-14T23:03:45Z",
  "nodeRef": {
    "kind": "Node",
    "name": "gce-node-47lts",
    "uid": "51546457-7026-11e8-b23f-42010a8a0002"
  },
  "providerStatus": null
}

Release note:

Now populate the associated node's UID in the machine.status.nodeRef field. 

@kubernetes/kube-deploy-reviewers

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: spew

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [spew]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[spew]

/assign @roberthbailey @k4leung4 @mkjelland

[roberthbailey]

/lgtm