Uses payload template value

Fixes #11

Adjusts to use template and tag values in message payload as defined by site specific custom Drupal module using `message_broker_producer_request()`.

Related:
Mandrill `send-template` API:  https://mandrillapp.com/api/docs/messages.JSON.html#method=send-template
https://github.com/DoSomething/dosomething/pull/3387