json.dumps the output from Python actions [structured data support]

* Fix: Update pack_mgmt/download.py to not call json.dumps

```
(virtualenv)~/stanley git:structured_data_pythonrunner ❯❯❯ st2 run packs.install packs=aws                                                                                                                ⏎ ✭ ✱ ◼
.................
+-----------------+---------------------------------------------------------+
| Property        | Value                                                   |
+-----------------+---------------------------------------------------------+
| id              | 548214c40640fd62612a9768                                |
| context         | {                                                       |
|                 |     "user": "lakshmi"                                   |
|                 | }                                                       |
| parameters      | {                                                       |
|                 |     "packs": [                                          |
|                 |         "aws"                                           |
|                 |     ]                                                   |
|                 | }                                                       |


| status          | failed                                                  |


| start_timestamp | 2014-12-05T20:25:40.792000Z                             |
| result          | {                                                       |
|                 |     "download": {                                       |
|                 |         "exit_code": 0,                                 |
|                 |         "result": {                                     |
|                 |             "aws": "Success."                           |
|                 |         },                                              |
|                 |         "stderr": "",                                   |
|                 |         "stdout": ""                                    |
|                 |     },                                                  |
|                 |     "virtualenv_prerun": {                              |
|                 |         "exit_code": 0,                                 |
|                 |         "result": [                                     |
|                 |             "aws"                                       |
|                 |         ],                                              |
|                 |         "stderr": "",                                   |
|                 |         "stdout": ""                                    |
|                 |     },                                                  |
|                 |     "setup_virtualenv": {                               |
|                 |         "exit_code": 1,                                 |
|                 |         "result": null,                                 |
|                 |         "stderr":                                       |
|                 | "st2.actions.python.SetupVirtualEnvironmentAction:      |
|                 | DEBUG    Setting up virtualenv for pack                 |
|                 | \"aws\"\nTraceback (most recent call last):\n  File \"/ |
|                 | home/lakshmi/stanley/st2actions/st2actions/runners/pyth |
|                 | on_action_wrapper.py\", line 106, in <module>\n         |
|                 | obj.run()\n  File \"/home/lakshmi/stanley/st2actions/st |
|                 | 2actions/runners/python_action_wrapper.py\", line 57,   |
|                 | in run\n    output = action.run(**self._parameters)\n   |
|                 | File \"/opt/stackstorm/packs/packs/actions/pack_mgmt/se |
|                 | tup_virtualenv.py\", line 45, in run\n                  |
|                 | self._setup_pack_virtualenv(pack_name=pack_name)\n      |
|                 | File \"/opt/stackstorm/packs/packs/actions/pack_mgmt/se |
|                 | tup_virtualenv.py\", line 72, in                        |
|                 | _setup_pack_virtualenv\n                                |
|                 | os.makedirs(self._base_virtualenvs_path)\n  File \"/hom |
|                 | e/lakshmi/stanley/virtualenv/lib/python2.7/os.py\",     |
|                 | line 157, in makedirs\n    mkdir(name, mode)\nOSError:  |
|                 | [Errno 13] Permission denied:                           |
|                 | '/opt/stackstorm/virtualenvs/'\n",                      |
|                 |         "stdout": ""                                    |
|                 |     }                                                   |
|                 | }                                                       |
| action          | packs.install                                           |
| callback        |                                                         |
+-----------------+---------------------------------------------------------+
(virtualenv)~/stanley git:structured_data_pythonrunner ❯❯❯
```
We (manas and I) explicitly decide to let it throw. We want to know what other string can through.