Geo: Don't attempt to sync repositories if their shard is not configured
On the GitLab Geo testbed, we only have one repository shard configured. Currently, we use most of our replication slots trying to replicate projects that will never succeeds.
We can exclude projects from consideration for replication in two situations:
- The shard isn't configured at all in gitlab.yml (not the case for us)
- The shard has been detected as failing by the storage circuit breaker