Write a cop for rubocop when adding postgresql attribute

We share a template between Postgresql and Geo-postgresql. This is causing failures in case we forget to include the attribute in geo section.

We have a few options:

1. Use separate templates -> I don't like this because it duplicates the effort of maintaining 2 templates
2. Always check if the variable is nil in the template -> I don't like this because we can still easily miss adding a new attribute to geo
3. Write a custom cop that will check if the same attribute exists in geo-postgresql section

@ibaum @brodock @stanhu What do you think about this?

I'm not sure rubocop is the correct place for this. Since we use chefspec our tests do have awareness of chef attributes. We could have a test that enumerates over default['gitlab']['postgresql'] and ensures there is a corresponding entry under default['gitlab']['geo-postgresql']. I believe there are a few instances where geo-postgresql just references the postgresql attributes, so there would need to be some sort of whitelist.

Out of curiosity, the geo-postgresql recipe seems to be a copy of the postgresql recipe, plus some extra. Is there a reason we don't just call the postgresql recipe with the geo-postgresql attributes set?

@marin wouldn't the suggestion made here: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2192 fix this by cloning the default attributes from postgresql?

@brodock Sorry, I completely forgot that discussion. That will indeed be the easiest solution...