Move flask app configuration outside python package

Move configuration outside the package hierarchy. Makes it a bit easier to find. Also, when coupled with the fact that the config root path can be provided in the environment, this makes it possible to produce a source package that is not tied to a given environment.
    
Make configuration hierarchical. This allows common configuration to be set in a root config while retaining the ability to override it in a specific app.
    
Set the GOVUK_ENV environment variable to test in unittests (this is taken from @alexmuller's commit e1224c4)
    
This commit is backwards compatible, the legacy config location is used as that is still where alphagov-deployment is putting it.