contrib/systemd: Add tmpfiles.d file

[cgwalters]

Now that that the apiserver can auto-generate self-signed
certificates, and drop them in /var/run/kubernetes (which is really
/run/kubernetes), we need to ensure it's created on boot.

(I'm not sure why the default isnt' persistent self-signed
certificates, but that's a different patch)

[cgwalters]

/cc @eparis

[erictune]

@cgwalters @eparis what are your plans for this PR? May I close until you are ready to come back to it?

[eparis]

@cgwalters I know on RHEL/Fedora /run/ == /var/run/. But if kube is specifically looking in /var/run, shouldn't we be creating the dir in /var/run instead of in /run, just in case someone somehow has a setup where they are not the same?

[cgwalters]

Any systemd-using system has /run -> /var/run. And systemd-tmpfiles only exists on systemd systems. Anyone packaging Kubernetes for an old system would need to use /var/run, but they'd do that in their packaging/init script.

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[cgwalters]

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