ceph-docker-common: do not log inside the container

Logging inside the container is not useful since it writes to the
overlayfs partition, resulting in potential performance degradation on
the container.

If you need to check the logs, just look at journald.

Signed-off-by: Sébastien Han <seb@redhat.com>

roles/ceph-docker-common/defaults/main.yml

@@ -1 +1,4 @@
 ---
+ceph_conf_overrides:
+  global:
+    log_file: /dev/null