[COOK-3505] Improvement to specify the administrator account name if different from 'Administrator'

ref. http://tickets.opscode.com/browse/COOK-3505

By default, the Windows account used as SysAdmin is 'Administrator', but on the http://www.vagrantbox.es/ boxes, it is named 'vagrant', so the installation crashes.
This improvement suggests providing a way to specify the Windows account name in the config file, for example:
```ruby
default_attributes(
  "sql_server" => {
    "accept_eula" => true,
    "sql_admin_accounts" => 'vagrant'
  }
```