Tests for DNS integration

Add test coverage for the integration with designate, confirming that
the correct DNS records are generated for

- floating IP created with dns_(domain|name) attributes
- instances that have a floating IP assigned

Add neutron-tempest-plugin-designate-scenario job, this will run the new
tests with Neutron DNS integration enabled.

Depends-On: Ib380d8a98e991a475b20140f5c37e3747aa5fc0c
Change-Id: Ib56531952cb70d8878631bbc0cb74a9b652d8c3a
diff --git a/neutron_tempest_plugin/api/base.py b/neutron_tempest_plugin/api/base.py
index 8db5108..8cd44ee 100644
--- a/neutron_tempest_plugin/api/base.py
+++ b/neutron_tempest_plugin/api/base.py
@@ -69,6 +69,9 @@
             force_new=force_new
         )
         # Neutron uses a different clients manager than the one in the Tempest
+        # save the original in case mixed tests need it
+        if credential_type == 'primary':
+            cls.os_tempest = manager
         return clients.Manager(manager.credentials)
 
     @classmethod