Make migration script log correctly

In all environments except dev, the django config overrides the logging.
It only does this when we access a variable from settings, which is the
first time we actually use any of django. Once django has bootstrapped
the logging, we can override with our own logging configuration - it
should be safe to set this to stdout as this is only ever run as a
script.
Standardise logging

No need to setup a class specific logger - use the module level one that
is already setup.
Merge pull request #241 from alphagov/make-migration-script-log-properly

Make migration script log correctly