Add arg parsing to datagov harvesting logic

[rshewitt]

User Story

In order to use harvest.py as a command line utility in cloud foundry ( via cf run-task), datagov wants to add command line argument parsing to datagov harvesting logic

Acceptance Criteria

[ACs should be clearly demoable/verifiable whenever possible. Try specifying them using BDD.]

• [ ] GIVEN datagov harvesting logic
  WHEN the arg parsing function is called
  THEN the passed arguments will be in a structured format easy to use by harvest.py

Background

we intend to use harvest.py as our runner code. the runner should accept a job_id. in order to do that we need to add an argument parsing library.

Security Considerations (required)

[Any security concerns that might be implicated in the change. "None" is OK, just be explicit here!]

Sketch

• implement argparse
• create function for parsing incoming arguments
• create test verifying arg parsing function works as intended. [Notes or a checklist reflecting our understanding of the selected approach]

[rshewitt]

harvest.py doesn't implement the arg parsing function. that'll happen in #4715. i reworded this ticket.

[btylerburton]

resolved with #4715