Refactors pgo-backrest Util

[andrewlecuyer]

Refactors the pgo-backrest util as used to exec into containers and run pgBackRest commands. This includes better organizing the logic for obtaining configuration settings and forming the pgBackRest command. Additionally, now includes the ability to verify the hashes of the pgBackRest conf.d directory before running the command, while now also allowing for the discovery of the Pod to be exec'd into via labels.

Checklist:

• [x] Have you added an explanation of what your changes do and why you'd like them to be included?
• [ ] Have you updated or added documentation for the change, as applicable?
• [x] Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

What is the current behavior? (link to any open issues here)

What is the new behavior (if this is a feature change)?

Other information:

[andrewlecuyer]

Thanks @cbandy, feedback addressed.