[ansible/product-demos] Stick to role prefix variable naming

The current variable naming of the roles does not stick with the recommended role naming of ansible-lint. All variables that need a rename have been identified using the following comment: ``` # noqa var-naming[no-role-prefix] - TODO : we should rework roles to use variable prefix, until scope is defined, silence is the way ``` This issue address the renaming of those and the associated tests & documentation.