Parameterizes options for admin listener in default recipe

People that really use this feature need to be able to tweak the configs. For example, in a wrapper cookbook you could reset the options:

```ruby
default['haproxy']['admin']['options'] = [
  'stats enable',
  'stats admin if TRUE',
  'stats uri /',
  'stats refresh 5s',
  'monitor-uri /monitor'
]
```

or leverage encrypted data:

```ruby
creds = Chef::EncryptedDataBagItem.load('credentials', 'haproxy')
node['haproxy']['admin']['options'] <<  "stats auth #{creds['username']}:#{creds['password']}"
```