Added support for Before and PartOf within unit section of custom systemd configuration

[glitchcrab]

What does this PR do?

• Further extends the fields available for templating systemd units.
• Switch systemd Service ExecStart to an array to support multiple commands (added in Enable templating within systemd unit files #351)

What is the effect of this change to users?

None, backwards compatible addition which is configured by provider charts.

How does it look like?

N/A

Any background context you can provide?

See #351

What is needed from the reviewers?

Do the docs need to be updated?

Should this change be mentioned in the release notes?

• CHANGELOG.md has been updated (if it exists)

[taylorbot]

Hey @glitchcrab, a test pull request has been created for you in the cluster-aws repo! Go to pull request giantswarm/cluster-aws#879 in order to test your cluster chart changes on AWS.

[glitchcrab]

ok I'm done adding stuff, this is ready to be reviewed please 🙏

[glitchcrab]

I decided that I might as well add support for EnvironmentFile while I'm at it too.

[github-actions]

There were no differences in the rendered Helm template.

Output

=== Differences when rendered with values file helm/cluster/ci/test-required-values.yaml ===

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=== Differences when rendered with values file helm/cluster/ci/test-zot-mc-and-local-values.yaml ===

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=== Differences when rendered with values file helm/cluster/ci/test-zot-mc-values.yaml ===

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=== Differences when rendered with values file helm/cluster/ci/test-zot-only-local-values.yaml ===

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=== No differences at all ===