Use cgi response code as error check point

[gabrielfernandes-codes]

Context

The motivation of this pull request are the following:

1 - For some reason, the response from cgi-fcgi had a different header size in case of the path was non-existent, in this way, even 404 responses weren't failing, since tail +5 was suppressing the response message as well.
2 - Checking only the response message could reduce the customizability of the script, that also could be used to check other endpoint instead only the fpm-status page, so in case this healthz page would reply something as 400, 500 or 503 for example, it wouldn't be possible to fail the health check.

In this way, the changes done substituted a single check for failed responses message, to a verification of failed response codes, in compliance of Kubernetes probe failure requirements.(https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/)

Changes

• Tests included
• Documentation updated
• Commit message is clear

[renatomefi]

Hello @fernandesGabriel,

Thanks for this contribution, I think it's a good addition to the script!

I added some comments, and could you please also add tests for the change?

Thanks again!

[gabrielfernandes-codes]

Hi @renatomefi

Sorry for the long time to reply you. I will take a look on your messages, as well as why the tests haven't passed.

[gabrielfernandes-codes]

@renatomefi I have updated this branch as my user was associated with my work email. Also made some minor adjustments. Please review and let me know if there is anything I should change. Tests are commented out as it would require a #38 to make it work properly.