Prevent crash and rule stacking

This is a regression to #2.

Using `ufw::databag`, Chef crashes on initial run because `node.firewall.rules` is not set:

```

================================================================================
Recipe Compile Error in /var/cache/chef/cookbooks/ufw/recipes/databag.rb
================================================================================


NoMethodError
-------------
Undefined node attribute or method `concat' on `node'. To set an attribute, use `concat=value' instead.


Cookbook Trace:
---------------
  /var/cache/chef/cookbooks/ufw/recipes/databag.rb:53:in `block in from_file'
  /var/cache/chef/cookbooks/ufw/recipes/databag.rb:47:in `each'
  /var/cache/chef/cookbooks/ufw/recipes/databag.rb:47:in `from_file'


Relevant File Content:
----------------------
/var/cache/chef/cookbooks/ufw/recipes/databag.rb:

 46:
 47:  rlist.each do |entry|
 48:    Chef::Log.debug "ufw::databag: \"#{entry}\""
 49:    if fw_db.member?(entry)
 50:      #add the list of firewall rules to the current list
 51:      item = data_bag_item('firewall', entry)
 52:      rules = item['rules']
 53>>     node.set['firewall']['rules'].concat(rules) unless rules.nil?
 54:    end
 55:  end
 56:
 57:  #now go apply the rules
 58:  include_recipe "ufw::default"
 59:
```

This pull request also addresses an issue where `rules` will stack after each run.