Expose Mistral outputs in the get/list workflows API

A request to add the "output" of a workflow to the following APIs:

GET /v2/workflows/
GET /v2/workflows/<workflowIdentifier>

Similar to the "input" that is returned today.

For example, for the following workflow:

---
version: '2.0'

wokflow_name:
  type: direct
  output:
    outputKey1: value1
    outputKey2:
        key2a: value2a
        key2b: value2b

  tasks:
    task1:
      action: std.async_noop

The response will be:

{
    "definition":"---\nversion: '2.0'\n\nwokflow_name:\n type: direct\n\n tasks:\n task1:\n action: std.noop\n",
    "name":"BDD_SimpleWF_dTg3iM",
    "tags":[

    ],
    "created_at":"2016-07-28 12:38:56",
    "updated_at":null,
    "scope":"private",
    "input":"",
    "project_id":"<default-project>",
    "id":"b38f81e6-7ff2-4717-9927-e999e89cff43",
    "output": [ outputKey1, outputKey2 ]
}