test: Overwrite existing taint when labeling nodes with NoSchedule

[tgraf]

Martynas reported that the following error occurs:

      unable label nodes: unable to taint node with 'kubectl taint nodes k8s3 key=value:NoSchedule': Exitcode: 1
      Stdout:
      Stderr:
         error: node k8s3 already has key taint(s) with same effect(s) and --overwrite is false

.. when the option --cilium.provision=false is used

Fixes: 014fd8e ("test: Decouple node labeling from node names")

[maintainer-s-little-helper]

Please set the appropriate release note label.

[maintainer-s-little-helper]

Please set the appropriate release note label.

[maintainer-s-little-helper]

Please set the appropriate release note label.

[maintainer-s-little-helper]

Please set the appropriate release note label.

[tgraf]

test-me-please

[brb]

Thanks!

[brb]

Tested locally - fixes the issue, ta!

[coveralls]

Coverage increased (+0.009%) to 44.608% when pulling 9f8b105 on pr/tgraf/ci-add-overwrite-to-noschedule-taint into 2950ea4 on master.