CORD-3148 Allow OpenStackServiceInstance not attached to a node

Change-Id: I0c6b0bdc15b775e6bff959a18f1e4731578a5da4
diff --git a/xos/synchronizer/steps/sync_openstackserviceinstance.py b/xos/synchronizer/steps/sync_openstackserviceinstance.py
index 9db35de..5e0b058 100644
--- a/xos/synchronizer/steps/sync_openstackserviceinstance.py
+++ b/xos/synchronizer/steps/sync_openstackserviceinstance.py
@@ -89,9 +89,9 @@
             # TODO(smbaker): No ssh keys specified
 
             if not instance.node:
-                raise Exception("No node specified for instance")
-
-            availability_zone="nova:%s" % instance.node.name
+                availability_zone = "nova"
+            else:
+                availability_zone="nova:%s" % instance.node.name
 
             log.info("Creating Instance", instance=instance, image_id=image_id, flavor_id=flavor_id,
                      availability_zone=availability_zone,