Use $() instead of ``

There are other instances of this in other files, just tidying
ceilometer at the moment.

Change-Id: I136a20d7ac50fdf02cbd1102613e324e313b7b0a
diff --git a/lib/ceilometer b/lib/ceilometer
index 9bb3121..26c698c 100644
--- a/lib/ceilometer
+++ b/lib/ceilometer
@@ -178,7 +178,7 @@
     configure_auth_token_middleware $CEILOMETER_CONF ceilometer $CEILOMETER_AUTH_CACHE_DIR
 
     if [ "$CEILOMETER_BACKEND" = 'mysql' ] || [ "$CEILOMETER_BACKEND" = 'postgresql' ] ; then
-        iniset $CEILOMETER_CONF database connection `database_connection_url ceilometer`
+        iniset $CEILOMETER_CONF database connection $(database_connection_url ceilometer)
         iniset $CEILOMETER_CONF DEFAULT collector_workers $API_WORKERS
     else
         iniset $CEILOMETER_CONF database connection mongodb://localhost:27017/ceilometer