Add "purge_zones" option to delete unmanaged zones

[tacerus]

PR progress checklist (to be filled in by reviewers)

• [x] Changes to documentation are appropriate (or tick if not required)
• [ ] Changes to tests are appropriate (or tick if not required)
• [ ] Reviews completed

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What type of PR is this?

Primary type

• [ ] [build] Changes related to the build system
• [ ] [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ ] [ci] Changes to the continuous integration configuration
• [x] [feat] A new feature
• [ ] [fix] A bug fix
• [ ] [perf] A code change that improves performance
• [ ] [refactor] A code change that neither fixes a bug nor adds a feature
• [ ] [revert] A change used to revert a previous commit
• [ ] [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [x] [docs] Documentation changes
• [ ] [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No, option is disabled by default.

Related issues and/or pull requests

n/a

Describe the changes you're proposing

This introduces a "purge_zones" toggle which, if enabled, ensures zones not managed using the firewalld pillar get deleted.

Pillar / config required to test the proposed changes

firewalld:
  purge_zones: true

Debug log showing how the proposed changes work

Documentation checklist

• [ ] Updated the README (e.g. Available states).
• [x] Updated pillar.example.

Testing checklist

• [ ] Included in Kitchen (i.e. under state_top).
• [ ] Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• [ ] Updated the relevant test pillar.

Additional context

Unsure how to have the test suite create a dummy file the absence of which could then be asserted with the pillar option enabled.