Pull system_metadata for notifications on instance.save()
Pull system_metadata for notifications on instance.save()

When saving changes on an instance object a notification is sent out.
An attempt is made to pull flavor information from the instance but if
system_metadata is not joined then an exception is raised and the
notification fails.  This ensures that system_metadata is available for
those notifications.

Change-Id: Ifd317f847c3839cd6019038bb3dc856b9f107000
Closes-bug: 1244311

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects