St2action logging changes.

* By default st2actions logs at INFO. DEBUG is rather verbose and only used in case of debugging or dev setup.
* At info logging there should be sufficient information to monitor the system via logs. An operator should be able to see when an execution starts, finishes and result of the execution.

Sample of what log of an execution would look like -
```
2014-12-12 02:35:50,082 139940152673296 INFO base [-] Dispatching Action to runner
{
    "status": "running",
    "start_timestamp": "2014-12-12 02:35:50.051000+00:00",
    "parameters": {
        "packs": [
            "jira"
        ]
    },
    "callback": {},
    "result": {},
    "context": {
        "user": "stanley"
    },
    "action": "packs.download",
    "id": "548a54860640fd5c456bc67e",
    "end_timestamp": null
}
2014-12-12 02:35:50,097 139940152673296 AUDIT access [-] Access granted to user "stanley" with the token set to expire at "2014-12-13T02:35:50.096835Z".
2014-12-12 02:36:02,537 139940152673296 AUDIT base [-] ActionExecution complete.
2014-12-12 02:36:02,538 139940152673296 INFO base [-] result :
{
    "result": {
        "jira": "Success."
    },
    "exit_code": 0,
    "stderr": "",
    "stdout": ""
}.
```