Update mysql.rb

(apologies in advance for the poor commit message)

My most recent attempt to spin up a drupal-lamp instance results in a compile error of the mysql recipe. Relevant log output:
```
================================================================================

Recipe Compile Error in /tmp/vagrant-chef-1/chef-solo-1/cookbooks/drupal/recipes/mysql.rb

================================================================================




Chef::Exceptions::ImmutableAttributeModification

------------------------------------------------

Node attributes are read-only when you do not specify which precedence level to set. To set an attribute use code like `node.default["key"] = "value"'




Cookbook Trace:

---------------

  /tmp/vagrant-chef-1/chef-solo-1/cookbooks/drupal/recipes/mysql.rb:31:in `from_file'



Relevant File Content:
----------------------
/tmp/vagrant-chef-1/chef-solo-1/cookbooks/drupal/recipes/mysql.rb:

 24:  include_recipe "database"
 25:  passwords = data_bag_item('users', 'mysql')[node.chef_environment]
 26:  Chef::Log::debug "drupal::mysql passwords = #{passwords.inspect}"
 27:
 28:  if Chef::Config[:solo]
 29:    Chef::Log.debug "drupal::mysql Setting chef solo node mysql passwords."
 30:    node.set['mysql']['server_debian_password'] = passwords["debian"] unless passwords["debian"].nil?
 31>>   node[:mysql][:server_root_password] = passwords["root"] unless passwords["root"].nil?
 32:    node.set['mysql']['server_repl_password'] = passwords["replication"] unless passwords["replication"].nil?
 33:  end
 34:
 35:  gem_package "mysql" if node[:db][:driver] == "mysql"
 36:
 37:  mysql_connection_info = {
 38:    :host => node[:db][:host],
 39:    :username => node[:db][:root],
 40:    :password => node[:mysql][:server_root_password]
```

Updating the node attribute to node.default as suggested by the log output seems to allow chef to get past this particular issue.