Fixes failed notification when deleting instance

In the notifier plugin for nova, the class Instance that translates instance_ref
returned by conduct.API to nova_client like Instance is not very robust, which
results some attributes are none or missing, and then causes pollsters raise
AttributeError or NoneTypeError

In this change, add the missing or none attributes to the Instance class.

Change-Id: I3e5aa6382021633a6bf7240bf58d3c3bed074785
Fixs: bug #1206731
Merge "Fixes failed notification when deleting instance"