Support `google_analytics_institutions_dimension_index` config var

A new variable will be added in AtoM 2.5.x to track institutions visits in Google Analytics. It's value will normally be `1`, but it should not have a default value, similar to [`atom_app_google_analytics_api_key`](https://github.com/artefactual-labs/ansible-atom/blob/master/defaults/main.yml#L99).