enhance doc regarding to manually delete cluster

[huxiaoliang]

fix #493

What this PR does / why we need it:

create a k8s cluster with 1 master node and 1 worker node, then manually delete this cluster according to doc here , but it doesn't work, this is due to no label like set=node and set=master on the Machine resource.
Fixes #493

Special notes for your reviewer:

None

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[ncdc]

LGTM (I think 😄) but you'll need to update your commit message so it doesn't say fix.

/ok-to-test

[huxiaoliang]

/retest

[sbueringer]

/approve

looks good. Seems to be an issue with go get'ing kustomize. I'll take a look later today

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: huxiaoliang, sbueringer

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[sbueringer]

@huxiaoliang This PR should fix the tests: #495

When this PR is merged you only have to rebase onto master to fix the tests.

[sbueringer]

/retest

[sbueringer]

@huxiaoliang Okay so rebase wasn't necessary for the tests because Prow tests a combination of master and a PR. But you have to change the commit message.

[huxiaoliang]

@sbueringer @ncdc invalid commit message get removed just now, thanks for your review.

[sbueringer]

Thx :)

/lgtm