Make NovaObject.get() avoid lazy-load when defaulting

Right now, NovaObject.get() will not properly emulate the dict
get() operation where a default value is returned if unset.

This makes it do that, and avoid attempting a lazy-load in the case
where a default is passed for a not-loaded value.

Related to blueprint compute-api-objects

Change-Id: I298d44b82591c14e877b56744eace5d9ddb93198
Merge "Make NovaObject.get() avoid lazy-load when defaulting"