Use the core v1 API instead of v1beta1, for GKE>=1.12 compatibility

[nolar]

Issue : #76

Starting from GKE 1.12 (Google Kubernetes Engine), the legacy APIs are disabled by default — disable-legacy-endpoints=true is added on the cluster creation. Official doc.

As such, events of v1beta1 are not accessible, and Kopf fails on the simple examples (see #76). v1 events should be used instead.

This PR switches to the v1 events. And v1beta1 are now disabled for Minikube, so it will fails if accidentally used. Also, it adds the unit-tests that were absent since #71 .

Beside the tests, I have manually tested the Kopf example operator to run in GKE — now, it works (before the fix, it was failing on the event posting).

[zincr]

🤖 zincr found 0 problems , 0 warnings

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[smileisak]

I was working on it. I did not see the issue 👍. Nice work !

[nolar]

@smileisak Oops. Sorry.

Since contributors started to join, I think, we should start syncing the allocation of issues by assigning them. I've self-assigned those I work on right now. Feel free to choose a free one: https://github.com/zalando-incubator/kopf/issues (or create a new one if the bug/enhancement is not mentioned yet).