cluster: improve SELinux compatibility

[dveeden]

What problem does this PR solve?

• Improve SELinux compatibility
• Fix the context of the systemd files after moving them from /tmp to /etc/systemd

This doesn't change the SELinux policy. We could do that later if we want. So no docs changes for now.

Because of this we don't change the tiup cluster check for SELinux.

What is changed and how it works?

Check List

Tests

Tested:

• Deploy cluster

• Start/Stop cluster

• Manual test (add detailed scripts or steps below)

Release notes:

NONE

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[dveeden]

/component cluster

[dveeden]

/label component/cluster

[ti-chi-bot]

@dveeden: The label(s) component/cluster cannot be applied. These labels are supported: help wanted, good first issue.

In response to this:

/label component/cluster

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