Fix output of `st2 execution get <id> -j`

In https://github.com/StackStorm/st2tests/pull/121, I had to write a few extra hacky steps to parse output of st2 execution get, because I was having issues parsing the output from the -j flag:

~$ st2 run examples.mistral-basic cmd=date
..
id: 5a31e81d32ed352825f47f77
action.ref: examples.mistral-basic
parameters:
  cmd: date
status: succeeded
result_task: task1
result:
  failed: false
  return_code: 0
  stderr: ''
  stdout: Thu Dec 14 02:55:26 UTC 2017
  succeeded: true
start_timestamp: Thu, 14 Dec 2017 02:55:25 UTC
end_timestamp: Thu, 14 Dec 2017 02:55:28 UTC
+--------------------------+------------------------+-------+------------+-------------------------------+
| id                       | status                 | task  | action     | start_timestamp               |
+--------------------------+------------------------+-------+------------+-------------------------------+
| 5a31e81d32ed352825f47f7a | succeeded (1s elapsed) | task1 | core.local | Thu, 14 Dec 2017 02:55:25 UTC |
+--------------------------+------------------------+-------+------------+-------------------------------+
~$ st2 execution get 5a31e81d32ed352825f47f77 -j
{
    "action": {
        "ref": "examples.mistral-basic"
    },
    "end_timestamp": "2017-12-14T02:55:28.108712Z",
    "id": "5a31e81d32ed352825f47f77",
    "parameters": {
        "cmd": "date"
    },
    "result": {
        "failed": false,
        "return_code": 0,
        "stderr": "",
        "stdout": "Thu Dec 14 02:55:26 UTC 2017",
        "succeeded": true
    },
    "result_task": "task1",
    "start_timestamp": "2017-12-14T02:55:25.087795Z",
    "status": "succeeded"
}
[
    {
        "action": "core.local",
        "id": "5a31e81d32ed352825f47f7a",
        "start_timestamp": "2017-12-14T02:55:25.711121Z",
        "status": "succeeded (1s elapsed)",
        "task": "task1"
    }
]

The output isn't valid JSON, so I couldn't directly use the asserts pack to parse the output. I had to use an intermediate step. This is invalid because it's both an object and a list next to each other. The object is the details of the execution, the list is the details of the subexecutions. This is kind of useful but only if presented in the right way.

I'd suggest either a flag to hide the list, or nest the list inside the object under a "subexecutions" key (the latter of which is breaking).

TODO

• Ensure the output of st2 execution get <id> -j produces valid JSON that can be safely read by other tools
• Change the inquiry tests in https://github.com/StackStorm/st2tests so that these hacky steps aren't included

Start with the st2 execution get <id> -j command and the inquiry tests in the linked st2tests pull request. Reproduce the output shown in this issue and inspect how the execution details and subexecution list are emitted. Done means the command produces valid JSON that other tools can parse directly, with the subexecutions represented consistently.