Added support for custom packages for Sensu's backend.

[kovukono]

Pull Request Checklist

Is this in reference to an existing issue? No.

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

New Features

• [ ] Added a Testing Artifact as either an automated test or a manual artifact on the PR.

• [X] Adedd documentation for it to the README.md

Purpose

Since this cookbook uses packagecloud to manage the repository for packages, it's currently impossible to install a non-commercial version of Sensu Go. This PR allows for specification of a yum or apt repository for Sensu Go packages built from source. By default, the commercial distribution is still the chosen.

Known Compatibility Issues

[derekgroh]

LGTM

[kovukono]

@majormoses Would it be possible to take a look at this?