write json to a more suitable directory

slapos/recipe/packer/template/packer_run.in

@@ -105,7 +105,7 @@ def getSocketStatus(host, port):
 def gen_packer_json(build):
   import json
   packer_json = {"builders": [build]}
-  with open(instance_dir + "packer.json", 'w') as outfile:
+  with open(instance_dir + "/packer.json", 'w') as outfile:
     json.dump(packer_json, outfile)