Add neutron-tempest-plugin-api job

The old tempest tests have been removed from the neutron repo, so
legacy-neutron-dsvm-api doesn't run anything at all anymore. Use the new
api job from the neutron-tempest-plugin repo instead.

Change-Id: I2d1854109474cee1923b9bffd75c3224a6f2b4bf

.zuul.yaml

@@ -16,3 +16,5 @@
         - tempest-full-queens-py3:
             branches: ^(?!driverfixes/).*$
             voting: false
+        - neutron-tempest-plugin-api:
+            voting: false