Ignore ufw error resulting from known bug on Debian 7

In order to check the version of the linux distribution we need to
set `gather_facts` to True.

Closes #73.

roles/common/tasks/ufw.yml

 - name: Enable ufw
   command: ufw --force enable
   when: "ufw_status.stdout.startswith('Status: inactive')"
+  register: ufw_enable
+  # ignore error resulting from known bug on Debian 7.0
+  failed_when: ufw_enable|failed and not ansible_lsb['codename'] == 'wheezy'

site.yml

 - hosts: all
   user: deploy
   sudo: True
-  gather_facts: False
+  gather_facts: True
   vars_files:
     - vars/defaults.yml
     - vars/user.yml