feat(config): support httpresponses in backend

[tacerus]

PR progress checklist (to be filled in by reviewers)

• Changes to documentation are appropriate (or tick if not required) (no existing docs)
• Changes to tests are appropriate (or tick if not required) (no existing tests)
• 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
• [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?

No. Merely the indentation of existing "http-request" lines in backend sections will change.

Related issues and/or pull requests

Describe the changes you're proposing

Pillar / config required to test the proposed changes

haproxy:
  backends:
    foo:
      httpresponses:
        - del-header:
            - server

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

Also switches httprequests to the shorter macro based rendering variant whilst at it.