RFR: get_results action for mistral workflow

Example of get_workflow_results action:

```
(virtualenv)~/stanley git:mistral_get_results â¯â¯â¯ st2 run mistral.get_workflow_results execution_id=aba76487-22a9-4d7a-80b7-2148312da36e
.
+-----------------+--------------------------------------------------------------+
| Property        | Value                                                        |
+-----------------+--------------------------------------------------------------+
| id              | 5494ac5c0640fd4a28014601                                     |
| context         | {                                                            |
|                 |     "user": "lakshmi"                                        |
|                 | }                                                            |
| parameters      | {                                                            |
|                 |     "execution_id": "aba76487-22a9-4d7a-80b7-2148312da36e"   |
|                 | }                                                            |
| status          | succeeded                                                    |
| start_timestamp | 2014-12-19T22:53:16.667000Z                                  |
| result          | {                                                            |
|                 |     "result": {                                              |
|                 |         "tasks": [                                           |
|                 |             {                                                |
|                 |                 "wf_name": "mistral.heyhey.heyhey",          |
|                 |                 "name": "say-greeting",                      |
|                 |                 "created_at": "2014-12-19 20:56:22",         |
|                 |                 "updated_at": "2014-12-19 20:56:25",         |
|                 |                 "state": "SUCCESS",                          |
|                 |                 "result": "{"greet": "$.localhost.stdout"}", |
|                 |                 "output": "{"task": {"say-greeting":         |
|                 | {"greet": "$.localhost.stdout"}}, "greet":                   |
|                 | "$.localhost.stdout"}",                                      |
|                 |                 "input": "{"ref": "mistral.greet",           |
|                 | "parameters": {"cmd": "3"}}",                                |
|                 |                 "id":                                        |
|                 | "3b5b7900-3fb6-4e4e-b077-4c97640d776a",                      |
|                 |                 "execution_id": "aba76487-22a9-4d7a-         |
|                 | 80b7-2148312da36e"                                           |
|                 |             },                                               |
|                 |             {                                                |
|                 |                 "wf_name": "mistral.heyhey.heyhey",          |
|                 |                 "name": "say-friend",                        |
|                 |                 "created_at": "2014-12-19 20:56:25",         |
|                 |                 "updated_at": "2014-12-19 20:56:25",         |
|                 |                 "state": "SUCCESS",                          |
|                 |                 "result": "{"towhom":                        |
|                 | "$.localhost.stdout"}",                                      |
|                 |                 "output": "{"towhom": "$.localhost.stdout",  |
|                 | "task": {"say-friend": {"towhom": "$.localhost.stdout"}}}",  |
|                 |                 "input": "{"ref": "mistral.friend",          |
|                 | "parameters": {"cmd": "noone"}}",                            |
|                 |                 "id": "77cfbc52-4c3e-49fa-                   |
|                 | 8f56-76f511b3df50",                                          |
|                 |                 "execution_id": "aba76487-22a9-4d7a-         |
|                 | 80b7-2148312da36e"                                           |
|                 |             },                                               |
|                 |             {                                                |
|                 |                 "wf_name": "mistral.heyhey.heyhey",          |
|                 |                 "name": "callback_on_success",               |
|                 |                 "created_at": "2014-12-19 20:56:25",         |
|                 |                 "updated_at": "2014-12-19 20:56:25",         |
|                 |                 "state": "SUCCESS",                          |
|                 |                 "result": "null",                            |
|                 |                 "output": "{"task": {"callback_on_success":  |
|                 | null}}",                                                     |
|                 |                 "input": "{"state": "SUCCESS", "result":     |
|                 | "$.localhost.stdout, $.localhost.stdout"}",                  |
|                 |                 "id": "df14c8d6-2eae-                        |
|                 | 41be-9743-c23fa8f6ff6a",                                     |
|                 |                 "execution_id": "aba76487-22a9-4d7a-         |
|                 | 80b7-2148312da36e"                                           |
|                 |             }                                                |
|                 |         ]                                                    |
|                 |     },                                                       |
|                 |     "exit_code": 0,                                          |
|                 |     "stderr": "",                                            |
|                 |     "stdout": ""                                             |
|                 | }                                                            |
| action          | mistral.get_workflow_results                                 |
| callback        |                                                              |
| end_timestamp   | 2014-12-19T22:53:17.015000Z                                  |
+-----------------+--------------------------------------------------------------+
(virtualenv)~/stanley git:mistral_get_results â¯â¯â¯
```