Enable script_security in keepalived

When `enable_script_security` is not present in keepalived.conf,
the daemon will print a warning in the journal:

    SECURITY VIOLATION - scripts are being executed but
    script_security not enabled.

This patch enables script security which ensures that a non-root
user cannot maliciously alter a script that keepalived may be running
as root.

This fixes Launchpad bug 1742487:

  https://bugs.launchpad.net/openstack-ansible/+bug/1742487

defaults/main.yml

@@ -49,3 +49,8 @@ cache_timeout: 600
 keepalived_instances: []
 keepalived_sync_groups: {}
 keepalived_bind_on_non_local: False
+
+# Any strings provided here will appear in the `global_defs` secction of the
+# keepalived configuration file.
+keepalived_global_defs:
+  - enable_script_security

templates/keepalived.conf.j2

@@ -13,6 +13,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+{% if keepalived_global_defs is defined %}
+global_defs {
+{% for def in keepalived_global_defs %}
+  {{ def }}
+{% endfor %}
+}
+{% endif %}
+
 {% for name, sync_group in keepalived_sync_groups.items() %}
 vrrp_sync_group {{ name }} {
   group {