Automatic merge from submit-queue (batch tested with PRs 45268, 47573, 47632, 47818)

NODE_TAINTS in gce startup scripts

Currently there is now way to pass a list of taints that should be added on node registration (at least not in gce or other saltbased deployment). This PR adds necessary plumbing to pass the taints from user or instance group template to kubelet startup flags. 

```release-note
Taints support in gce/salt startup scripts. 
```

The PR was manually tested. 
```
NODE_TAINTS: 'dedicated=ml:NoSchedule'
```
in kube-env results in 
```
spec:
[...]
  taints:
  - effect: NoSchedule
    key: dedicated
    timeAdded: null
    value: ml
```

cc: @davidopp @gmarek @dchen1107 @MaciekPytel