adding a returns to the backend init

[tarcinil]

fixes #82

Pull Request Checklist

fixes #82

General

• [X] Update Changelog following the conventions laid out here

• [X] Update README with any necessary configuration snippets

• [X] Cookstyle (rubocop) passes

• [X] Foodcritic passes

• [X] Rspec (unit tests) passes

• [X] Inspec (integration tests) passes

Purpose

This allows the sensu-backend init to accept a return code of 3 as an acceptable return. It is returned by the application when the backend is already init'd. As Sensu Go does not have a status endpoint for the init, it is not possible to idempotently check, so they return 3 as the alternative.

[majormoses]

@tarcinil thanks for the fix!