* Fixes a policy check on doing the zone export that caused a 403
unless the tenant was an admin
* Adds ZoneFile and ZoneFileRecord models to parse a zone file
* Adds some "meta" tests for the new models
* Updates the test_export_domain test to check the zone file text
Change-Id: I65c93f5d5283fc5962f4a2bf0ac8abae1966e6d6