The Facts class and subclasses no longer take a dict argument for
instantiation.  populate() now returns self.facts.
Other changes:
- Facts.__init__() takes over most of the work from populate() so that
  subclasses can benefit from its knowledge.
- Drop setting unknown facts to 'NA' in __init__() in various
  subclasses.
- Check for presence of facts in get_virtual_facts()
- Update ansible_facts() to use facts.update(<classname>().populate())