Fix initialization order in Django app

[bmermet]

This fix #304

I have tested locally that it fixes the bug, but I haven't added a unit test since I haven't found a simple way to do it. It requires running the unit tests with a different config at startup and I'm not convinced it's worth adding new test envs only to check on this.

[palazzem]

Thank you! About the test, as we've discussed "offline", you can write a test reloading the application in that way:

from django.apps import apps

def test_service_not_called(self):
    # mock/spy the tracer so that you know if the service endpoint has been called or not

    # change our local settings because they've been initialized and the Django decorator will not work in this case
    settings.ENABLED = False
    # reload the application
    app = apps.get_app_config('datadog_django')
    app.ready()
    # now you can check if the service method has been called or not

Also it's worth looking if with this approach ^^ you should restore and re-initialize the app. If this is the case, could be interesting to write a decorator for that so it can be reused. I'd say to do that in another PR later on if we have to.

[palazzem]

since the test is nothing_is_written, you can add a tracer.trace() call to create and finish a Span. Calling app.ready() will only send services and not traces, so adding something could be great. If you prefer, you can split the test in two: one for services and one for traces.

[palazzem]

Perfect!! Good to me! Good job!