There is code in oslo.versionedobjects that expects an object
class to have this method:
_from_db_object(context, item_cls(), db_item, **extra_args)
To conform to this, this patch adds 'context' as the first
parameter of base.IronicObject._from_db_object().
As a bonus, some docstrings were cleaned up.
Change-Id: I09b02f47d47cec22d03460b693f9e5c488cf04c7
Closes-Bug: #1670778