refactor(ynh_backup): follow good practises for var naming / syntax

Follow this example: https://lab.frogg.it/lydra/yunohost/clients-offre-ynh/-/merge_requests/40

added To Do label

assigned to @arthur.boudreault

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added backup enhancement labels

created branch 83-refactor-ynh_backup-follow-good-practises-for-var-naming-syntax to address this issue

mentioned in merge request !60 (closed)

removed To Do label

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added Need review ynh_backup labels and removed backup enhancement labels

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closed

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