Start aligning Admin app with config styles used elsewhere

- no config overrides - now all set in environment
- use different files for staging and live too allow for differently named env variables
- updates to run_app and run_tests scripts to set correct environment (test/development) so correct config picked up
- use environment file on deployed environments to pick correct config

tests/conftest.py

@@ -24,7 +24,7 @@ from notifications_python_client.errors import HTTPError
 
 @pytest.fixture(scope='session')
 def app_(request):
-    app = create_app('test')
+    app = create_app()
 
     ctx = app.app_context()
     ctx.push()