rgw.yaml: keep the client.0 key even if empty (firefly)

Because rgw.py iterates over it to run the rgw server. If it is removed
the rgw servers are not run and all fails.

Signed-off-by: Loic Dachary <loic@dachary.org>
(cherry picked from commit 5cfcb76860302386375ca0cf1c7a960ef6659a75)