fix: use django-environ for django DEBUG settings (GoogleCloudPlatform#6792)

## Description

Django examples use `django-environ` but it is not being used for the `DEBUG` settings.

## Checklist
- [x] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [x] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [x] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [x] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [x] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: luigibertaco

appengine/flexible/django_cloudsql/mysite/settings.py

@@ -58,7 +58,7 @@
 
 # SECURITY WARNING: don't run with debug turned on in production!
 # Change this to "False" when you are ready for production
-DEBUG = True
+DEBUG = env("DEBUG")
 
 # SECURITY WARNING: App Engine's security features ensure that it is safe to
 # have ALLOWED_HOSTS = ['*'] when the app is deployed. If you deploy a Django

appengine/standard_python3/django/mysite/settings.py

@@ -57,7 +57,7 @@
 
 # SECURITY WARNING: don't run with debug turned on in production!
 # Change this to "False" when you are ready for production
-DEBUG = True
+DEBUG = env("DEBUG")
 
 # SECURITY WARNING: App Engine's security features ensure that it is safe to
 # have ALLOWED_HOSTS = ['*'] when the app is deployed. If you deploy a Django