Add nodeset ubuntu-bionic-2-node for bionic multinode testing

Given that Bionic is the Ubuntu LTS release standard for the Stein
cycle, there should be a 2-node nodeset for it.  This will enable
multinode grenade testing on python3 to be done correctly.

Change-Id: I4a756f1f284a6967853da2dc34e477b2ba7b2340
Needed-By: https://review.openstack.org/622612

zuul.d/nodesets.yaml

@@ -47,6 +47,18 @@
       - name: ubuntu-bionic
         label: ubuntu-bionic
 
+- nodeset:
+    name: ubuntu-bionic-2-node
+    nodes:
+      - name: primary
+        label: ubuntu-bionic
+      - name: secondary
+        label: ubuntu-bionic
+    groups:
+      - name: subnodes
+        nodes:
+          - secondary
+
 - nodeset:
     name: ubuntu-bionic-arm64
     nodes: