Fix 'properties' is undefined errors

When following the example playbook for the role, it is possible for
properties undefined to be raise. Because default() filters don't work
with undefined dictionary keys, ensure a default dictionary for
properties exists.

Change-Id: Iee2e992efe8ee801506e5de622bd90ac3915a33c
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yml

@@ -162,3 +162,6 @@ lxc_container_veth_wiring: false
 
 # Enable systemd-resolved
 lxc_container_enable_resolved: true
+
+# Dictionary of settings for containers
+properties: {}