Update ubuntu-xenial-2-node to match centos-7-2-node

In the other places subnodes is a group and there is a node called
secondary put in to it. It shouldn't matter, but as centos-7-2-node and
ubuntu-xenial-2-node are defined differently it could cause someone to
be confused or think there is a reason for the divergence.

Change-Id: Icd28da7c1c20b605841cd27f4f656e4b8f812fee

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = linters
+envlist = linters,bindep
 
 [testenv]
 basepython = python3

zuul.d/nodesets.yaml

@@ -91,8 +91,12 @@
     nodes:
       - name: primary
         label: ubuntu-xenial
-      - name: subnodes
+      - name: secondary
         label: ubuntu-xenial
+    groups:
+      - name: subnodes
+        nodes:
+          - secondary
 
 - nodeset:
     name: ubuntu-xenial-3-node
@@ -201,8 +205,12 @@
     nodes:
       - name: primary
         label: ubuntu-xenial
-      - name: subnodes
+      - name: secondary
         label: ubuntu-xenial
+    groups:
+      - name: subnodes
+        nodes:
+          - secondary
 
 - nodeset:
     name: legacy-ubuntu-xenial-3-node
@@ -217,4 +225,4 @@
       - name: subnodes
         nodes:
           - secondary-1
-          - secondary-2
+          - secondary-2