Create 8GB file on subnodes

In CI we have /root/swapfile of 8GB on every node mounted as swap,
and it's critical for TripleO deployments which fails without it
because of memory lack.
Create such file on every subnode.

Change-Id: Id2e6237c603320eefb7a76512dbb544263614c71

roles/nodepool-setup/defaults/main.yml

@@ -8,3 +8,4 @@ repos:
   - https://git.openstack.org/openstack-dev/devstack
   - https://git.openstack.org/openstack-infra/devstack-gate
 zuul_changes: "{{ lookup('env', 'ZUUL_CHANGES') }}"
+subnode_swap_size: 8388608 # 8GB

roles/nodepool-setup/tasks/main.yml

@@ -2,6 +2,15 @@
 - include: etc-nodepool.yml
   become: true
 
+- name: Create and enable swap on subnode
+  shell: >
+    dd if=/dev/zero of=/root/swapfile bs=1024 count={{ subnode_swap_size }};
+    chmod 600 /root/swapfile;
+    mkswap /root/swapfile;
+    swapon /root/swapfile;
+    echo "/root/swapfile swap swap defaults 0 0" >> /etc/fstab;
+  become: true
+
 - name: Create /etc/ci directory
   file:
     path: /etc/ci