Prior to this commit the metadata.json file for this module listed OSs that are not supported by puppet agent as supported.
This commit aims to refactor the metadata.json file to only list OSs that are supported by puppet agent.

The list of supported OSs can be found here:
https://puppet.com/docs/pe/2021.7/supported_operating_systems.html

(MAINT) Removing support for CentOS 8

Revert "(MAINT) Removing support for CentOS 8"

Changes made to ensure that no malformed commands are passed through to the system.

- Fixes made to spec tests
- onlyif commands corrected
- remove unnecessary shell_escapes
- variable used in place of replicated assignment
- $_file_path neatened

The unless variable in an exec manifest, which $verify_command is directly passed too, accepts either an Array or and Array of Arrays as input.
Updating the variables type and default values to match this.
In the case where it is passed a String it proceeds to wrap it in an Array and then treat it as such.

(maint) Codebase Hardening

Release prep v8.2.1