ActiveRecord::RecordNotSaved creating settings when Rails cache not functional
When using foreman_memcache and it's unconfigured and not functional, then a new setting is added (e.g. installing a new plugin), app startup simply fails with "ActiveRecord::RecordNotSaved".
To reproduce, install foreman_memcache and another plugin which adds settings, then Foreman should fail to start.
Rails.cache.delete returns 'false' when the cache isn't functional. The Setting#clear_cache before_save callback then calls cache.delete, returning the result. This causes the saving of the setting to fail entirely.