WIP: Training Microservice (METS validation)

[ross-spencer]

This commit adds a new METS validation microservice which serves a training purpose around how to develop a basic microservice.

The microservice will reject invalid METS generated by the create METS V2 scripts in the workflow.

TODO

• Remove create AIP reminder.
• Excess validation code in write_mets function.

Connected to archivematica/Issues#153
Connected to archivematica/Issues#655
Connected to archivematica/Issues#1129

Black-box tests for this work

@black-box 
Feature: Alma wants to ensure that METS files generated by Archivematica are well-formed and valid.

  Scenario: METS validation passes
    Given a "zipped bag" transfer type located in "SampleTransfers/BagExamples/ZippedBags/BagTransfer.zip"
	When the transfer is approved
    Then the "Verify bag, and restructure for compliance" job completes successfully
    Then the "Validate AIP METS" job completes successfully

  Scenario: METS validation fails
    Given a "zipped bag" transfer type located in "SampleTransfers/BagExamples/Invalid/InvalidBag.zip"
    When the transfer is approved
    Then the "Verify bag, and restructure for compliance" job completes successfully
    And the "Validate AIP METS" job fails

[qubot]

Can one of the admins verify this patch?