task/ta29 -- integrate agent_id check into loader

loader.py
* Agent Id should load into the "job" table as a foreign key to the
  "agent" table
* replaced method agent_exists() with get_agent_id() to better align
  with business rules
* JOB_MAP has a "Agent Id" key now uses a get_agent_id() as a callback
Merge branch 'feature/us21' into dev