* Zone object no longer treats them special, some tests needed adjusting
b/c of this, some provider's tests may also need adjusting, though
they should not plan changes since they won't (yet) have
SUPPORTS_ROOT_NS
* _process_desired_zone filters and warns when not supported
* YamlProvider supports them
* TinyDnsBaseSource supports them
Adds a new source requested in #239. This source allows a user to pull data from
a legacy system (Bind9, etc.) that does not have an API/existing provider via
AXFR Zone Transfer.