Issue3523 mbp international settings

Fixes #3523 

Moves from hard coded template setting in transactional payloads to dynamic setting by "Application ID" (Affiliate country code) and available templates in Mandrill.

- Adds admin setting for "Application ID" (Affiliate country code) that's connected to the same setting in `message_broker_producer` module.
- Adds admin setting for Mandrill API key.
- Uses cURL call to Mandrill API to lookup templates by  "Application ID" (Affiliate country code).
- Adds admin interface to manage template assignment to email transaction types: `user_registration`, `user_password`, `campaign_signup`, `campaign_group_signup` and `campaign_reportback` by "Application ID" (Affiliate country code) .
- Uses template assignments to define 'template` value in message payloads replacing hard code / US specific settings.

**To test**:
- go to `/admin/config/dosomething/dosomething_mbp`
  - Add "Application ID", ex: "CA". Currently only template entries for "CA" and "GB".
  - Add Mandrill Key setting
- Save form to be presented with "Transactional Template" settings for each transaction type
- Create test transaction to trigger transactional messages using selected template.
This seems a little dangerous..  Have you tested this code with transactionals from a clean staging DB?

Will this break things when the variables are not set (as will be the case when this code is deployed)? Do we need an install hook to set the relevant variables?