Make libvirt function on CentOS

Recent regressions in the code base removed permission setting of
/dev/kvm to root:kvm 660 permissions which are default for CentOS's
version of libvirt.

Also Libvirt must be able to read its cnofiguration file, which was
previously 600 root:root.  Now its 644 root:root so its always readable.
This is fine, since this file doesn't contain any secret information.

Change-Id: Id87cf5da8e37bc5bb613ce919d0293803d0fe5ed
Closes-Bug: #1500733

ansible/roles/nova/templates/nova-libvirt.json.j2

@@ -5,7 +5,7 @@
             "source": "/opt/kolla/config_files/libvirtd.conf",
             "dest": "/etc/libvirt/libvirtd.conf",
             "owner": "root",
-            "perm": "0600"
+            "perm": "0644"
         }{% if enable_ceph | bool %},
         {
             "source": "/opt/kolla/config_files/secrets",

docker/nova/nova-libvirt/start.sh

@@ -4,4 +4,12 @@ set -o errexit
 # Loading common functions.
 source /opt/kolla/kolla-common.sh
 
+# TODO(SamYaple): Tweak libvirt.conf rather than change permissions.
+# Fix permissions for libvirt
+# Do not remove unless CentOS has been validated
+if [[ -c /dev/kvm ]]; then
+    chmod 660 /dev/kvm
+    chown root:kvm /dev/kvm
+fi
+
 exec $CMD