Trunk 4446: Support deriving auto_expire_date from duration and duration units for Drug Orders

Changes for ticket https://issues.openmrs.org/browse/TRUNK-4446
It wasn't part of it initially. Made this change as per discussion in https://issues.openmrs.org/browse/TRUNK-4446?focusedCommentId=210286&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-210286