Support uppercased env variables

Problem/Motivation

Some hosting services insist that env variables should be uppercase *cough*platform.sh*cough*, which makes it impossible to target most container parameters, as they're lowercase per convention.

Proposed resolution

Add an optional evn var, DRUPAL_SERVICE_DOWNCASE_VARS, which when set will downcase the variable name before setting them, so that DRUPAL_SERVICE_CORS__CONFIG___ENABLED=1 will behave like DRUPAL_SERVICE_cors__config___enabled=1.