Add namespace option for events

[muff1nman]

By default node-problem-detector writes events to the default namespace. This PR adds an option to allow events to be written to a different namespace.

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[k8s-ci-robot]

Hi @muff1nman. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[xueweiz]

/ok-to-test

[xueweiz]

Looking at the unit test result, seems there is some problem:

GO111MODULE=on go list -mod vendor -tags " journald" ./... | \
	grep -v "./vendor/*" | \
	GO111MODULE=on xargs go vet -mod vendor -tags " journald"
# k8s.io/node-problem-detector/pkg/exporters/k8sexporter/problemclient
vet: pkg/exporters/k8sexporter/problemclient/problem_client_test.go:45:33: too few arguments in call to getNodeRef

Seems the refactoring is breaking some other code paths. Could you please fix them? Thanks! You can run make test to verify whether the problem has been fixed.

[xueweiz]

/lgtm
/approve

@muff1nman the patch looks good to me. However, it will only be merged after you signed the Linux Foundation CLA. Here are the necessary steps.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: muff1nman, xueweiz

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[muff1nman]

I signed it.