Run neutron-grenade job only for branches before Ussuri

This job is still running python 2.7. As we are dropping py2 support in
Ussuri cycle, lets drop this job from the integrated-gate template for
branches newer than Train.
There is same job called "grenade-py3" which runs on python 3 already
and this is still used in projects CI.

Change-Id: I93dc3c8352184d8f6516d2f31b3082bf1a3ab999

zuul.d/zuul-legacy-project-templates.yaml

@@ -51,14 +51,14 @@
             branches: ^stable/(newton|ocata)$
         - tempest-full
         - neutron-grenade:
-            branches: ^(?!stable/(newton|ocata)).*$
+            branches: ^stable/(pike|queens|rocky|stein|train).*$
     gate:
       jobs:
         - legacy-tempest-dsvm-neutron-full:
             branches: ^stable/(newton|ocata)$
         - tempest-full
         - neutron-grenade:
-            branches: ^(?!stable/(newton|ocata)).*$
+            branches: ^stable/(pike|queens|rocky|stein|train).*$
 
 - project-template:
     name: lib-forward-testing-plugin