Add bionic nodeset for legacy job to switch to bionic

We have migrated all the zuulv3 native jobs to Bionic[1]
and now time to make legacy job also start running on Bionic
node.

This commit adds the bionic nodeset (single as well as multinode) for
legacy job so that each project can start their testing.

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

Change-Id: Ie79b20b7fc5cec63be26d2ac8231f6822e61e089

zuul.d/nodesets.yaml

@@ -146,6 +146,12 @@
       - name: primary
         label: ubuntu-xenial
 
+- nodeset:
+    name: legacy-ubuntu-bionic
+    nodes:
+      - name: primary
+        label: ubuntu-bionic
+
 - nodeset:
     name: legacy-ubuntu-xenial-2-node
     nodes:
@@ -157,3 +163,15 @@
       - name: subnodes
         nodes:
           - secondary
+
+- nodeset:
+    name: legacy-ubuntu-bionic-2-node
+    nodes:
+      - name: primary
+        label: ubuntu-bionic
+      - name: secondary
+        label: ubuntu-bionic
+    groups:
+      - name: subnodes
+        nodes:
+          - secondary