[train]: add designate-tempest-plugin as req

Add designate-tempest-plugin to required-projects of
neutron-tempest-plugin-designate-scenario-train and pin it to 0.7.0.
This should avoid the client initialization issue bring in by [1].

[1]: https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/800291

Change-Id: Ib58f0cf251b84c21db0f4b8defd213c770739134
Closes-Bug: #1950346

zuul.d/train_jobs.yaml

@@ -231,7 +231,13 @@
     parent: neutron-tempest-plugin-designate-scenario
     nodeset: openstack-single-node-bionic
     override-checkout: stable/train
-    required-projects: *required-projects-train
+    required-projects:
+      - openstack/neutron
+      - name: openstack/neutron-tempest-plugin
+        override-checkout: 1.5.0
+      - openstack/tempest
+      - name: openstack/designate-tempest-plugin
+        override-checkout: 0.7.0
     vars:
       branch_override: stable/train
       network_api_extensions_common: *api_extensions