[COOK-4294] Wordpress database password not idempotent

Due to behaviour of ```node.set_unless```, which compares if node attribute is set only on normal level, user's password given in default attributes has been always ignored. For example this code:
```ruby
Chef::Log.debug(node['wordpress']['db']['pass'])

node.set_unless['wordpress']['db']['pass'] = secure_password
node.save unless Chef::Config[:solo]

Chef::Log.debug(node['wordpress']['db']['pass'])
```
Gave results:
```
[2014-02-01T15:24:07+00:00] DEBUG: secret
[2014-02-01T15:24:07+00:00] DEBUG: 3zvynXotnj2qAge5bkNY
```

Also user's password in MySQL has never been updated, so as a result, if user provisioned his machine twice, wordpress couldn't connect to database, which is an obvious violation of idempotency.

This pull request addresses both issues (even though database passwords should be fixed with {{database}} cookbook, as in COOK-4180).