Use a guard and not .error? method for sensu_ctl

[webframp]

I don't remember the reason for trying to use .error? to check the output of the shell exec. What matters though is that the configure is run once when needed and not every time once it's already configured.

This change switches to using a guard and a method to compare the configured backend url.

The shortcoming of this approach is that as the resource is currently written it supports a username and password property, however we do not have an implementation that can safely handle reconfiguring these values so they will only work the first time the resource is used.

Fixes #63

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Pull Request Checklist

Is this in reference to an existing issue?

General

• [ ] Update Changelog following the conventions laid out here

• [ ] Update README with any necessary configuration snippets

• [ ] Cookstyle (rubocop) passes

• [ ] Foodcritic passes

• [ ] Rspec (unit tests) passes

• [ ] Inspec (integration tests) passes

New Features

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

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

Purpose

Known Compatibility Issues

[webframp]

@majormoses just a start here to experiment with an alternate approach

[webframp]

This one still needs work and isn’t ready to be included for #80 yet