add optional size to container role with default

roles/lxc_guest/defaults/main.yaml

@@ -2,3 +2,4 @@
 auto_start: true
 container_state: started
 distro: jessie
+vm_size: 5G

roles/lxc_guest/tasks/main.yaml

@@ -13,6 +13,7 @@
       lxc_container:
           name: "{{ vm_name }}"
           backing_store: lvm
+          fs_size: "{{ vm_size }}"
           vg_name: "{{ inventory_hostname }}vg"
           lv_name: "vm_{{ vm_name }}"
           fs_type: xfs