nova-libvirt requires a /sys/fs/cgroup bindmount

Nova-compute is curerntly generating a backtrace when trying to nova
boot a VM.  The backtrace is described in the bug log.  Bind-mounting
/sys/fs/cgroup:/sys/fs/cgroup in the libvirt container resolves the
problem.

Change-Id: I5a56b61fd260728fb76e19bcbdf403b80a4ec5a6
Closes-Bug: #1490326

ansible/roles/nova/tasks/start.yml

@@ -18,6 +18,7 @@
     volumes:
       - "{{ node_config_directory }}/nova-libvirt/:/opt/kolla/nova-libvirt/:ro"
       - "/run:/run"
+      - "/sys/fs/cgroup:/sys/fs/cgroup"
       - "/var/lib/nova/instances"
     env:
       KOLLA_CONFIG_STRATEGY: "{{ config_strategy }}"