[Tags] Prevent duplicate tags in root volume of the head node.

[gmarciani]

Description of changes

Prevent duplicated tags in the head node launch template.

Why there were duplicates?
Because get_custom_tags() returns custom tags and a subset of default tags.
The long term fix would be to fix get_custom_tags() but since it is extensively used, this would imply a refactoring with a low ROI at the moment.

Tests

• Create a cluster with this change and verified that the tag parallelcluster:cluster-name is not duplicated in the head node root volume (it used to be before this change).

References

• Link to impacted open issues.
• Link to related PRs in other packages (i.e. cookbook, node).
• Link to documentation useful to understand the changes.

Checklist

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