Add legacy 3 nodes nodeset based on Ubuntu bionic

We are migrating all the legacy jobs running on Bionic nodesets.

This commit adds the bionic legacy nodeset of 3 nodes for ha jobs,
for example- neutron-tempest-dvr-ha-multinode-full

[1] http://lists.openstack.org/pipermail/openstack-discuss/2018-December/000837.html

Change-Id: Ieef62d7c733694a50707f9670ff370e6c7b90b44

zuul.d/nodesets.yaml

@@ -175,3 +175,18 @@
       - name: subnodes
         nodes:
           - secondary
+
+- nodeset:
+    name: legacy-ubuntu-bionic-3-node
+    nodes:
+      - name: primary
+        label: ubuntu-bionic
+      - name: secondary-1
+        label: ubuntu-bionic
+      - name: secondary-2
+        label: ubuntu-bionic
+    groups:
+      - name: subnodes
+        nodes:
+          - secondary-1
+          - secondary-2