fix(apiserver): allow multiple service-account-key-file

[hatappi]

What this PR does / why we need it:
Currently, an error occurs when multiple service-account-key-file are specified for kube-apiserver.
This PR will fix it.

Which issue(s) this PR fixes:
fixed #7645

Special notes for your reviewer:

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Hi @hatappi. Thanks for your PR.

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[hatappi]

/check-cla

[hatappi]

/assign @chrislovecnm

[rifelpet]

/ok-to-test

I confirmed the new test fails without the fix in place:

--- FAIL: Test_KubeAPIServer_BuildFlags (0.00s)
    ./nodeup/pkg/model/kube_apiserver_test.go:116: flags did not match.  actual="--insecure-port=0 --secure-port=0 --service-account-key-file=/srv/kubernetes/server.key,/srv/kubernetes/service-account.key" expected="--insecure-port=0 --secure-port=0 --service-account-key-file=/srv/kubernetes/server.key --service-account-key-file=/srv/kubernetes/service-account.key"
FAIL
exit status 1

and that the test's expected output is what kube-apiserver requires for the flag. Thanks @hatappi !
/lgtm

[justinsb]

Thanks @hatappi

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hatappi, justinsb

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