added 'image_name' property to vPGWC Tenant model
you can now choose which VM you want for a given instance
in tosca using the 'image_name' property

Change-Id: I2f889caf6640d96c18f2cb006d3270cd8cb4555a
(cherry picked from commit 4f45a4fb634186ae36a7bf6ea1a8817af4455562)
diff --git a/xos/synchronizer/steps/sync_vpgwc.py b/xos/synchronizer/steps/sync_vpgwc.py
index bbfc0a8..d9ec4f0 100644
--- a/xos/synchronizer/steps/sync_vpgwc.py
+++ b/xos/synchronizer/steps/sync_vpgwc.py
@@ -36,4 +36,7 @@
     # Gets the attributes that are used by the Ansible template but are not
     # part of the set of default attributes.
     def get_extra_attributes(self, o):
-        return {"display_message": o.display_message, "s5s8_pgw_tag": o.s5s8_pgw_tag}
+        return {"display_message": o.display_message, 
+                "s5s8_pgw_tag": o.s5s8_pgw_tag, 
+                "image_name": o.image_name,
+               }