add an ansible task to set DJANGO_SETTINGS_MODULE in wsgi handler script

this seems to be a shortcoming of mod_wsgi-express, which does not save
this var from the call to a line such as:
"./manage.py runmodwsgi --setup-only --settings=XXX"


Former-commit-id: 082479fde709b689a38e670927c367f0af29ca73