Refactor and document templating of passwords

What does this MR do?

Implement findings of #2999 (closed)

• Replace all uses of "<%= x.dump[1..-2] =>" with <%= x.to_json =>
• Replace all uses of "{% x | strings.TrimSpace %}" with {% x | strings.TrimSpace | strings.Squote %}
• Update any RSpecs impacted by the above
• Add note to doc/development/style_guide.md regarding templating passwords from files

Testing

1. Set a few of the secrets to include characters that must be escaped according to YAML, TOML, or JSON.
2. Ensure all containers render complete, valid configurations.
   • Handy linters: http://www.yamllint.com/, https://jsonlint.com/, https://www.toml-lint.com/

Related issues

Related to #1214 (closed), #1094 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened

Closes #2999 (closed)