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.

Conflicts:

	nova/objects/base.py

The conflict is from change Icd2944f17b6100d51007dbc48da90e4e992bbd48

Change-Id: Ifd317f847c3839cd6019038bb3dc856b9f107000
Closes-bug: 1244311
(cherry picked from commit e03963b1e42ca2900b108201e77edf2386c5c46a)