Fix compute_node stats

The compute_node object has several issues with the stats field
that are cleaned up by this patch, including the following:

- obj_make_compatible() should check the stats property exists
  before attempting to delete it
- from_db_object() should not create the stats property if it
  is not in the database field (it is a nullable field)
- create() should convert the stats property to its database
  field format if it is present
- tests for create() and save() should check that the database
  format conversion is done

This method does not change any fields so there is no change to
the object's version.

Change-Id: I5c68994d618f0ecf9409c37321bc87a006c431ac
Merge "Fix compute_node stats"