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Add support for unmanaged nodes for Azure cloud provider
**What this PR does / why we need it**:
Continue of [Azure cross resource groups feature](https://github.com/kubernetes/features/issues/604).
This PR adds support for unmanaged nodes (such as on-prem or on other clouds) that are labeled with `alpha.service-controller.kubernetes.io/exclude-balancer=true` and `kubernetes.azure.com/managed=false`. Azure cloud provider would exclude such nodes from LoadBalancer backends and always assumes they are existing.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
See KEP [here](https://github.com/kubernetes/community/blob/master/keps/sig-azure/0025-20180809-cross-resource-group-nodes.md).
**Special notes for your reviewer**:
Azure cloud provider won't provision network routes for on-prem nodes, so cluster admins should ensure the network (including pod-to-pod, pod-to-node and node-to-node connectivity) has been set up properly.
**Release note**:
```release-note
Azure cloud provider now supports unmanaged nodes (such as on-prem) that are labeled with `kubernetes.azure.com/managed=false` and `alpha.service-controller.kubernetes.io/exclude-balancer=true`
```
/assign @khenidak @andyzhangx
/sig azure
/kind feature
/milestone v1.12
8aea674681