Enable Strict mode to decode KubeOne API

[kron4eg]

What this PR does / why we need it:
To warn people at very early stages that they have some mistakes in their YAMLs

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #759

Does this PR introduce a user-facing change?:

Strict KubeOne config decoding

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[xmudrii]

Would it make sense to add a unit test to take the example configuration from pkg/cmd/config.go and try to parse it? In that case, we would know that example config and API are correct and that unmarshaling works as expected.

We can leave this for a follow-up, but I'm just wondering would that make any sense.

[kron4eg]

To me it would be "testing k8s.io/apimachinery/pkg/runtime/serializer" and why would we want it? So I avoided those tests.

[xmudrii]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: c98486690fc1e6e421ad39555f516e1e23297660