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

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>
Merge pull request #96 from dachary/wip-8988-default-idle-timeout

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