Filesystem circuit breaker ignores storages unless they are "enabled"

Summary

Per https://gitlab.com/gitlab-org/gitlab-ee/issues/3360#note_40661376

The application settings object stores a list of enabled shards. This is a subset of the shards in use, but the circuit breaker uses it, rather than the full list, when performing readiness checks.

Steps to reproduce

Start with a GitLab instance with two shards. Set new projects to be created only on shard A, then break shard B and run readiness / liveness checks against it.

What is the current bug behavior?

Shard B is ignored

What is the expected correct behavior?

Shard B should be marked as unavailable

/cc @reprazent