Add Instance.fault

This makes Instance load fault as an optional attribute, if
requested. I didn't implement save for this, as I'm not sure
we will (want to) support setting instance faults via that
interface.

Related to unified-object-model
Fixes bug 1192004

Change-Id: I581e1f49c4c593ece97cf32dc7cea3f7a7cbd953
Merge "Add Instance.fault"