Skip creating weave-passwd secret if it already exists

[xmudrii]

What this PR does / why we need it:

Running kubeone install on already provisioned cluster (e.g. to repair it) with WeaveNet CNI causes the following error:

INFO[14:54:35 CEST] Applying weave-net CNI plugin…               
WARN[14:54:36 CEST] Task failed, error was: failed to create weave-net Secret: secrets "weave-passwd" already exists

The reason for this error is that we try to unconditionally create the secret, even if it exists. This PR changes the behavior, so the secret is created only if it doesn't exist.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
xref #531

Does this PR introduce a user-facing change?:

Ensure weave-passwd secret is created on install only if it doesn't exist

/assign @kron4eg

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xmudrii

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[kubermatic-bot]

LGTM label has been added.

Git tree hash: 1a5bbe77054dd537f83b95d90ec42ee2afd363e7

[xmudrii]

/retest

[xmudrii]

/retest