Making workflow endpoint able to upload multiple workflows

* Added WorkflowListSpec that represents a top level specification
  for uploading workflows (shouldn't be confused with WorkflowSpecList)
* Added workflow service that takes care of parsing, validating and
  creating multiple workflows from definition
* Added unit tests for workflow service
* Fixed workflow controller to work with multiple workflows on post/put
* Fixed workflow controller tests

Change-Id: I70a46a16e1970673fec0352091e4b610f320fbb8
Merge "Making workflow endpoint able to upload multiple workflows"