Rewrite relative path in RepositoryReplicas conditionally

Rails is failing on RepositoryReplicas tests as it now checks the replica path. However, the Rails tests do not configure a database so Praefect can't get the replica path from there. It's not possible to mock out the function with the interface provided either. To fix the problem, this commit adds a config check which enables the relative path rewriting only if repository-specific primaries are enabled, which also indicates that a database must be configured.