Add obj_set_defaults() to NovaObject

This provides a way to set the default values on fields that have them.
Previously, defaults were only set on nullable fields when set to None,
which wasn't universally-useful.

Related to blueprint kilo-objects

Change-Id: Ie646c0363d4af22ec8a588a6a1567c30bab99ebb
Merge "Add obj_set_defaults() to NovaObject"