PR progress checklist (to be filled in by reviewers)

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

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
• [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

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

Does this PR introduce a BREAKING CHANGE?

Yes.

The formula was installing the Apache plugin by default
which is not reasonable. Also, as letsencrypt:pkgs is a list,
adding another entry in the list was not replacing this default, causing
undesired packages to be installed.

The new default is an empty list of packages, defaulting to certbot in
the code.

Related issues and/or pull requests

Describe the changes you're proposing

This PR defaults to just installing certbot by default, and use letsencrypt:pkgs to specify other packages, as plugins.

For most users of the formula, it won't require any change to their pillars, but marking it as a BREAKING CHANGE to play safe.

Pillar / config required to test the proposed changes

Debug log showing how the proposed changes work

Documentation checklist

• Updated the README (e.g. Available states).
• 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