Migrate gate job to py3

gate jobs are modified to run on py2 on stable
branch and on py3 on ussuri onwards.

Change-Id: Ie6e287d134523931418256180dc5c55fd32f0db6

.zuul.yaml

@@ -18,7 +18,7 @@
     name: devstack-plugin-nfs-tempest-full
     parent: tempest-full
     description: |
-      tempest-full job based on NFS.
+      tempest-full job based on NFS with py2.
       Former names for this job where:
         * legacy-tempest-dsvm-full-devstack-plugin-nfs
     timeout: 10800
@@ -36,35 +36,54 @@
     irrelevant-files:
       - ^(test-|)requirements.txt$
       - ^setup.cfg$
+    branches:
+      - stable/ocata
+      - stable/pike
+      - stable/queens
+      - stable/rocky
+      - stable/stein
+      - stable/train
+
+- job:
+    name: devstack-plugin-nfs-tempest-full
+    parent: tempest-full-py3
+    description: |
+      tempest-full-py3 job based on NFS.
+      Former names for this job where:
+        * legacy-tempest-dsvm-full-devstack-plugin-nfs
+    timeout: 10800
+    required-projects:
+      - opendev.org/x/devstack-plugin-nfs
+    host-vars:
+      controller:
+        devstack_plugins:
+          devstack-plugin-nfs: https://opendev.org/x/devstack-plugin-nfs
+    vars:
+      devstack_localrc:
+        ENABLE_NFS_CINDER: true
+        # enabled by default in tempest-full:
+        ENABLE_VOLUME_MULTIATTACH: false
+    irrelevant-files:
+      - ^(test-|)requirements.txt$
+      - ^setup.cfg$
+    branches: ^(?!stable/(ocata|pike|queens|rocky|stein|train)).*$
 
 - job:
     name: devstack-plugin-nfs-tempest-full-train
     parent: devstack-plugin-nfs-tempest-full
     override-checkout: stable/train
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: 'False'
 
 - job:
     name: devstack-plugin-nfs-tempest-full-stein
     parent: devstack-plugin-nfs-tempest-full
     override-checkout: stable/stein
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: 'False'
 
 - job:
     name: devstack-plugin-nfs-tempest-full-rocky
     parent: devstack-plugin-nfs-tempest-full
     override-checkout: stable/rocky
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: 'False'
 
 - job:
     name: devstack-plugin-nfs-tempest-full-queens
     parent: devstack-plugin-nfs-tempest-full
     override-checkout: stable/queens
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: 'False'