(no-ticket) Use --nomigrations by default

This shortens test setup time from 30sec to 1sec, yet the database
is correctly initialized by pytest-django using model definitions.

On Jenkins, --migrations is now specified explicitly.

Change-Id: I4421e3a4ea37f5383613b4daebcf0da680ce1d36

pytest.ini

@@ -8,6 +8,7 @@ python_files =
 python_functions =
 addopts =
     --html=test_report.html --css=test_report.css --self-contained-html
+    --nomigrations
 filterwarnings =
     ignore::django.utils.deprecation.RemovedInDjango110Warning
     ignore:DateTimeField:RuntimeWarning