change variables since settings.main used by manage.py uses DJANGO_*

[ecstt]

About installing from source:
the minimal example suggests in the env:

docs/docs/installation/from-source.rst

Lines 107 to 109 in 282339e

	# Application
	MEDIA_ROOT=/home/wger/media
	STATIC_ROOT=/home/wger/static

when using a different user than the suggested new 'wger' user and importing the wger.env for bootstrap command with
set -a; . /home/<non_default_folder>/wger.env; set +a
then checking the env:
manage.py diffsettings | grep MEDIA_ROOT
shows the value of 'MEDIA_ROOT' (and 'STATIC_ROOT') as not changed from the default used in settings.main.

In settings.main MEDIA_ROOT and STATIC_ROOT are:

MEDIA_ROOT = env.str('DJANGO_MEDIA_ROOT', '/home/wger/media')
STATIC_ROOT = env.str('DJANGO_STATIC_ROOT', '/home/wger/static')

so DJANGO_* is required to overwrite the values.
Running python manage.py collectstatic without this change in the env gave me an error since it tried to access the non existent /home/wger folder.