Update CRM_Utils_Constant::value to support env variables

Update CRM_Utils_Constant::value to support env variables

This is something I discussed with @totten earlier but as the rc was being cut at the time
we went for the more conservative approach of only adding env support to thhe debug_query
function.  This extends to all defines - meaning they can be defined at the script level e.

env CIVICRM_DEBUG_LOG_QUERY=backtrace drush cvapi

(that already works)