allow CENTRAL_CONTENT_DOWNLOAD_BASE_URL to be set in ENV
allow CENTRAL_CONTENT_DOWNLOAD_BASE_URL to be set in ENV

christianmemije/kolibri,jonboiser/kolibri,lyw07/kolibri,mrpau/kolibri,mrpau/kolibri,learningequality/kolibri,indirectlylit/kolibri,benjaoming/kolibri,lyw07/kolibri,christianmemije/kolibri,DXCanas/kolibri,jonboiser/kolibri,DXCanas/kolibri,indirectlylit/kolibri,indirectlylit/kolibri,learningequality/kolibri,benjaoming/kolibri,christianmemije/kolibri,mrpau/kolibri,learningequality/kolibri,DXCanas/kolibri,benjaoming/kolibri,DXCanas/kolibri,jonboiser/kolibri,christianmemije/kolibri,lyw07/kolibri,indirectlylit/kolibri,mrpau/kolibri,jonboiser/kolibri,benjaoming/kolibri,learningequality/kolibri,lyw07/kolibri