Switch ovn tempest jobs to nested virt nodes

These jobs taking much time and failing randomly
in tempest tests while ssh, test instances taking
much time to be up, Let's switch these jobs
to nested virt nodes which will help in avoiding
these issues.

Job results in previous patchset of this patch looks
much better with the switch.

Change-Id: I7f8dde56d28c5aba1ad39337ab60a041db14d16b

zuul.d/networking-ovn-jobs.yaml

@@ -1,3 +1,13 @@
+- nodeset:
+    name: ovn-nested-virt-ubuntu-bionic
+    nodes:
+      - name: controller
+        label: nested-virt-ubuntu-bionic
+    groups:
+      - name: tempest
+        nodes:
+          - controller
+
 - job:
     name: networking-ovn-base
     description: Base job for devstack/tempest based networking-ovn jobs.
@@ -253,6 +263,7 @@
     name: networking-ovn-tempest-dsvm-ovs-release
     description: Job testing for devstack/tempest testing networking-ovn with the latest released OVN branch
     parent: networking-ovn-base
+    nodeset: ovn-nested-virt-ubuntu-bionic
     vars:
       devstack_localrc:
         OVN_BRANCH: v20.03.0
@@ -262,6 +273,7 @@
     name: networking-ovn-tempest-dsvm-ovs-release-python2
     description: Job testing for devstack/tempest testing networking-ovn with the latest released OVN branch
     parent: networking-ovn-base
+    nodeset: ovn-nested-virt-ubuntu-bionic
     vars:
       # TODO(flaviof): remove trunk tests test from blacklist when bug
       # https://bugs.launchpad.net/tempest/+bug/1927985 and