Change older branches parent jobs and nodeset

Some recent changes introduced a branches variable into our yoga,
xena and wallaby jobs. Manila-tempest-plugin is branchless, so such
jobs should not have the branches variable assigned to them.

Change-Id: I8647d268ccd642def4a003f63ea26d804016c20f
diff --git a/zuul.d/manila-tempest-stable-jobs.yaml b/zuul.d/manila-tempest-stable-jobs.yaml
index 4188915..aa33c16 100644
--- a/zuul.d/manila-tempest-stable-jobs.yaml
+++ b/zuul.d/manila-tempest-stable-jobs.yaml
@@ -73,20 +73,26 @@
 
 - job:
     name: manila-tempest-plugin-lvm-yoga
-    parent: manila-tempest-plugin-lvm-stable
+    parent: manila-tempest-plugin-lvm-base
+    nodeset: openstack-single-node-focal
     override-checkout: stable/yoga
     vars:
       tempest_exclude_regex: "(^manila_tempest_tests.tests.scenario.*IPv6.*)"
+      <<: *manila_tempest_image_pinned_vars
 
 - job:
     name: manila-tempest-plugin-lvm-xena
-    parent: manila-tempest-plugin-lvm-stable
+    parent: manila-tempest-plugin-lvm-base
+    nodeset: openstack-single-node-focal
     override-checkout: stable/xena
+    vars: *manila_tempest_image_pinned_vars
 
 - job:
     name: manila-tempest-plugin-lvm-wallaby
-    parent: manila-tempest-plugin-lvm-stable
+    parent: manila-tempest-plugin-lvm-base
+    nodeset: openstack-single-node-focal
     override-checkout: stable/wallaby
+    vars: *manila_tempest_image_pinned_vars
 
 - project-template:
     name: manila-tempest-plugin-jobs-using-service-image-stable