Fix centos 7 DevStack CI job

- use 3 compute nodes on last OpenStack versions with CentOS due to an
  experienced lack of memory forbidding nova for spawning VMs

Change-Id: I54048672bc0e85710dd925c0255c1ee17a655796

zuul.d/nodes.yaml

@@ -66,3 +66,43 @@
          nodes:
            - compute1
            - compute2
+
+
+- nodeset:
+     name: openstack-four-node-centos-7
+     nodes:
+       - name: controller
+         label: centos-7
+       - name: compute1
+         label: centos-7
+       - name: compute2
+         label: centos-7
+       - name: compute3
+         label: centos-7
+     groups:
+       # Node where tests are executed and test results collected
+       - name: tempest
+         nodes:
+           - controller
+       # Nodes running the compute service
+       - name: compute
+         nodes:
+           - compute1
+           - compute2
+           - compute3
+       # Nodes that are not the controller
+       - name: subnode
+         nodes:
+           - compute1
+           - compute2
+           - compute3
+       # Switch node for multinode networking setup
+       - name: switch
+         nodes:
+           - controller
+       # Peer nodes for multinode networking setup
+       - name: peers
+         nodes:
+           - compute1
+           - compute2
+           - compute3

zuul.d/tobiko-devstack.yaml

@@ -163,7 +163,7 @@
 - job:
     name: tobiko-devstack-scenario-centos-7
     parent: tobiko-devstack-scenario
-    nodeset: openstack-three-node-centos-7
+    nodeset: openstack-four-node-centos-7
     voting: false
     vars:
       devstack_localrc:
@@ -175,6 +175,7 @@
     name: tobiko-devstack-scenario-centos-7-queens
     parent: tobiko-devstack-scenario-centos-7
     override-checkout: stable/queens
+    nodeset: openstack-three-node-centos-7
     voting: false
 
 
@@ -182,6 +183,7 @@
     name: tobiko-devstack-scenario-centos-7-rocky
     parent: tobiko-devstack-scenario-centos-7
     override-checkout: stable/rocky
+    nodeset: openstack-three-node-centos-7
     voting: false
 
 
@@ -189,6 +191,7 @@
     name: tobiko-devstack-scenario-centos-7-stein
     parent: tobiko-devstack-scenario-centos-7
     override-checkout: stable/stein
+    nodeset: openstack-three-node-centos-7
     voting: false