allow string keys in config hash

This makes the class more flexible and allows the use of attributes to initialize the handler.

Example:

```ruby
node.default['chef_client']['config']['report_handlers'] = [
  {
    'class' => 'Chef::Handler::Datadog',
    'arguments' => [
      'api_key' => node['datadog']['api_key'],
      'application_key' => node['datadog']['application_key'],
      'use_ec2_instance_id' => node['datadog']['use_ec2_instance_id']
    ]
  }
]
```