Enable the storage check by default on nodes that run unicorn
The storage check performs 1 request per second by default, and only one request at a time. So it should cause too much load.
These results are reported on the admin health page, but do not affect the circuit-breaker until that is enabled using a feature flag.
This would make the tests in https://gitlab.com/gitlab-com/infrastructure/issues/3403 as simple as flipping the feature flag.