Do not override DAEMON_ARGS through /etc/default files

After adding support we faced broken configuration for openstack
services (see related bug). OS services' init.d files "sources"
/etc/defaults/{service_name} files which could override DAEMON_ARGS.

Change-Id: I59b2130ace694149cb7a6d9c0a3ca88c8b0e8afd
Related-bug: PROD-17968
diff --git a/ceilometer/files/default b/ceilometer/files/default
index 857d54b..e09b535 100644
--- a/ceilometer/files/default
+++ b/ceilometer/files/default
@@ -1,4 +1,4 @@
 # Generated by Salt.
 {% if values.logging.log_appender %}
-DAEMON_ARGS="--log-config-append=/etc/ceilometer/logging/logging-{{ service_name }}.conf"
+DAEMON_ARGS="${DAEMON_ARGS} --log-config-append=/etc/ceilometer/logging/logging-{{ service_name }}.conf"
 {% endif %}