Use transformer config for tasks

Although there's an implicit dependency on backdrop.write's config (they
must be configured to use the same AMQP URL), it's too fragile to
depend on config deployed by a different fabric task.

We specify config both in tasks and the worker so that they can be more
easily split in future.

backdrop/transformers/tasks.py

@@ -1,12 +1,12 @@
 from celery import Celery
 
-# Load the appropriate config from backdrop.write
+# Load the appropriate config as a python module
 import importlib
 from os import getenv
 
 GOVUK_ENV = getenv("GOVUK_ENV", "development")
 config = importlib.import_module(
-    "backdrop.write.config.{}".format(GOVUK_ENV))
+    "backdrop.transformers.config.{}".format(GOVUK_ENV))
 
 app = Celery(
     'transformations',