lib/neutron: fix metering agent startup with systemd

With systemd, we now require absolute paths passed to run_process.

Change-Id: Iabd425a6d0ebaaaa5b13bc4a7b062158940958e6
diff --git a/lib/neutron b/lib/neutron
index 2ffabd4..a672d39 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -443,7 +443,7 @@
     fi
 
     if is_service_enabled neutron-metering; then
-        run_process neutron-metering "$NEUTRON_METERING_BINARY --config-file $NEUTRON_CONF --config-file $NEUTRON_METERING_AGENT_CONF"
+        run_process neutron-metering "$NEUTRON_BIN_DIR/$NEUTRON_METERING_BINARY --config-file $NEUTRON_CONF --config-file $NEUTRON_METERING_AGENT_CONF"
     fi
 }