Replace YAML.safe_load with Puppet::Util::Yaml.safe_load in docker_stack

[akremer]

Summary

Addresses #973. Using Puppet::Util::Yaml.safe_load rather than just YAML.safe_load already contains a check for this specific issue.

Note, there are numerous other issues with this method and file, (#968, #878, #848, etc). I will be proposing a separate update to fix those, but this is just a small atomic update to fix this specific issue.

Additional Context

#973 contains the additional context

Related Issues (if any)

Mention any related issues or pull requests.

Checklist

• [X] 🟢 Spec tests.
• [X] 🟢 Acceptance tests.
• [X] Manually verified. (For example puppet apply)

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