Workaround Rails filesystem ID tests in Praefect

Rails tests configure Praefect in front of the tests that exercise
the Rugged direct git access code. As Praefect is now deriving the
filesystem IDs from the names of the virtual storages, the filesystem
id checks fail and thus the test fail. This is not a problem in practice
as one wouldn't use rugged in a real-world setup with Praefect. This
commit worksaround the tests by returning the filesystem ID from the
Gitaly node if a virtual storage has only one Gitaly node configured.
This matches the setup the tests use and thus pass them. The workaround
and the filesystem ID code can be removed in 15.0 once the rugged patches
and NFS support are dropped.