Fixed tests and updated the provider schema to use a DefaultFunc

The default stack changed from ‘cedar’ to ‘cedar-14’, so updated the
acceptance tests to reflect this.

Updating the schema makes testing easier and gives you a way to
configure the provider using env variables. It also makes the provider
more inline following the TF 0.2 approach.
Merge pull request #579 from svanharmelen/f-update-heroku-provider

providers/heroku: fixed acc tests and updated the provider schema to use a DefaultFunc