Fixes devstack stackrc

One of the earlier patchset while updating stackrc
missed on adding "+=" while adding multiple services.

Closes-Bug: #1439983
Change-Id: I74f788e15b7da05a93fc8d99c562e51386b65053
diff --git a/stackrc b/stackrc
index 0d8f059..abedb00 100644
--- a/stackrc
+++ b/stackrc
@@ -49,9 +49,9 @@
     # Keystone - nothing works without keystone
     ENABLED_SERVICES=key
     # Nova - services to support libvirt based openstack clouds
-    ENABLED_SERVICES=,n-api,n-cpu,n-net,n-cond,n-sch,n-novnc
+    ENABLED_SERVICES+=,n-api,n-cpu,n-net,n-cond,n-sch,n-novnc
     # Glance services needed for Nova
-    ENABLED_SERVICES=,g-api,g-reg
+    ENABLED_SERVICES+=,g-api,g-reg
     # Cinder
     ENABLED_SERVICES+=,c-sch,c-api,c-vol
     # Dashboard