TMSS-234: POST API to copy Scheduling unit draft in scheduling set

The POST API/scheduling_set/{scheduling_set_id}/copy_scheduling_unit_draft/ allows to copy all the schedulign unit draft that belong to the specified scheduling set.
The user can optionally specify as post parameter  "copy_reason": "string", if not set the original copy_reason will be copied, is a wrong copy_reason is specified, copy_reason= None