Prettify action execution output in CLI

Add CLI options to "st2 execution get" for displaying execution result. By default, the output will print the status and result of the action execution as text. If given "-d" option, the command will print the full detail of the action execution in table format. If given "-k <key name>", the command will select the given key in the result dict and print the value. Dot notation for nested dicts is supported.

```
$ st2 execution get 547fb3691e2e24089440d49f
STATUS: succeeded
RESULT:
{
    "localhost": {
        "failed": false,
        "stderr": "",
        "return_code": 0,
        "succeeded": true,
        "stdout": "PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.015 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.024 ms
64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.030 ms

--- 127.0.0.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1998ms
rtt min/avg/max/mdev = 0.015/0.023/0.030/0.006 ms"
    }
}

$ st2 execution get 547fb3691e2e24089440d49f -j
{
    "status": "succeeded",
    "result": {
        "localhost": {
            "failed": false,
            "stderr": "",
            "return_code": 0,
            "succeeded": true,
            "stdout": "PING 127.0.0.1 (127.0.0.1) 56(84) bytes of
data.\n64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.015 ms\n64
bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.024 ms\n64 bytes from
127.0.0.1: icmp_seq=3 ttl=64 time=0.030 ms\n\n--- 127.0.0.1 ping
statistics ---\n3 packets transmitted, 3 received, 0% packet loss, time
1998ms\nrtt min/avg/max/mdev = 0.015/0.023/0.030/0.006 ms"
        }
    }
}

$ st2 execution get 547fb3691e2e24089440d49f -d
+-----------------+--------------------------------------------------------------+
| Property        | Value                                                        |
+-----------------+--------------------------------------------------------------+
| id              | 547fb3691e2e24089440d49f                                     |
| action          | core.ping                                                    |
| callback        |                                                              |
| context         | {                                                            |
|                 |     "user": "stanley"                                        |
|                 | }                                                            |
| parameters      | {                                                            |
|                 |     "cmd": "127.0.0.1 3"                                     |
|                 | }                                                            |
| result          | {                                                            |
|                 |     "localhost": {                                           |
|                 |         "failed": false,                                     |
|                 |         "stderr": "",                                        |
|                 |         "return_code": 0,                                    |
|                 |         "succeeded": true,                                   |
|                 |         "stdout": "PING 127.0.0.1 (127.0.0.1) 56(84) bytes   |
|                 | of data.                                                     |
|                 | 64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.015 ms     |
|                 | 64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.024 ms     |
|                 | 64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.030 ms     |
|                 |                                                              |
|                 | --- 127.0.0.1 ping statistics ---                            |
|                 | 3 packets transmitted, 3 received, 0% packet loss, time      |
|                 | 1998ms                                                       |
|                 | rtt min/avg/max/mdev = 0.015/0.023/0.030/0.006 ms"           |
|                 |     }                                                        |
|                 | }                                                            |
| start_timestamp | 2014-12-04T01:05:45.039000Z                                  |
| status          | succeeded                                                    |
+-----------------+--------------------------------------------------------------+

$ st2 execution get 547fb3691e2e24089440d49f -dj
{
    "status": "succeeded",
    "start_timestamp": "2014-12-04T01:05:45.039000Z",
    "parameters": {
        "cmd": "127.0.0.1 3"
    },
    "callback": {},
    "result": {
        "localhost": {
            "failed": false,
            "stderr": "",
            "return_code": 0,
            "succeeded": true,
            "stdout": "PING 127.0.0.1 (127.0.0.1) 56(84) bytes of
data.\n64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.015 ms\n64
bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.024 ms\n64 bytes from
127.0.0.1: icmp_seq=3 ttl=64 time=0.030 ms\n\n--- 127.0.0.1 ping
statistics ---\n3 packets transmitted, 3 received, 0% packet loss, time
1998ms\nrtt min/avg/max/mdev = 0.015/0.023/0.030/0.006 ms"
        }
    },
    "context": {
        "user": "stanley"
    },
    "action": "core.ping",
    "id": "547fb3691e2e24089440d49f"
}

$ st2 execution get 547fb3691e2e24089440d49f -k localhost.stdout
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.015 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.024 ms
64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.030 ms

--- 127.0.0.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1998ms
rtt min/avg/max/mdev = 0.015/0.023/0.030/0.006 ms

$ st2 execution get 547fb3691e2e24089440d49f -k localhost.stdout -j
{
    "result": {
        "localhost": {
            "stdout": "PING 127.0.0.1 (127.0.0.1) 56(84) bytes of
data.\n64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.015 ms\n64
bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.024 ms\n64 bytes from
127.0.0.1: icmp_seq=3 ttl=64 time=0.030 ms\n\n--- 127.0.0.1 ping
statistics ---\n3 packets transmitted, 3 received, 0% packet loss, time
1998ms\nrtt min/avg/max/mdev = 0.015/0.023/0.030/0.006 ms"
        }
    }
}
```
Merge pull request #865 from StackStorm/STORM-842/cli-exec-output

Prettify action execution output in CLI