[systemd] Use a private /tmp directory

[jbudz]

This creates an isolated tmp directory for the kibana service. Reads
and writes to /tmp will end up in /tmp/systemd-private-*-kibana.service-*/tmp,
isolated to the current process.

Reference: https://www.freedesktop.org/software/systemd/man/systemd.exec.html#PrivateTmp=

In practice, after #112198 I believe the only remaining use of os.tmpdir() in production is by the legacy log rotation feature.

I tested access by setting:

logging.rotate:
  enabled: true
  usePolling: true
logging.dest: /var/log/kibana/kibana.log

I tailed logs until setup was done and there were no errors:

{"type":"log","@timestamp":"2021-09-15T21:33:44-05:00","tags":["warning","logging:rotate"],"pid":159674,"message":"Looks like
 your current environment support a faster algorithm than polling. You can try to disable `usePolling`"}

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[jbudz]

@elasticmachine merge upstream

[jbudz]

@elasticmachine merge upstream

[tylersmalley]

@elasticmachine merge upstream

[kibanamachine]

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• continuous-integration/kibana-ci/pull-request
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@elasticmachine merge upstream