feat(config): allow to specify `dc_mailname_in_oh`

[javierbertoli]

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

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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
• [X] [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No.

Related issues and/or pull requests

Describe the changes you're proposing

This PR allows to specify the internal config option dc_mailname_in_oh that is set, at least in Debian's family, by the maintainer scripts when the output of /etc/mailname is added to the dc_other_hostnames config option.

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).
• [X] Updated pillar.example.

Testing checklist

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

Additional context