[RFR] Trigger visbility improvements

- Adds a cli command to returns the Trigger for a TriggerType.
- In cli TriggerType = Trigger & Trigger = TriggerSpecification. However, this is handled via aliasing and not in the type system.

(Btw, name is open for review)

```
$ st2 trigger getspecs core.st2.webhook
+--------------------------+------+------+----------------+------------------+
| id                       | name | pack | parameters     | type             |
+--------------------------+------+------+----------------+------------------+
| 5490a7dceba55b4b05a1c614 | 1    | core | {u'url': u'1'} | core.st2.webhook |
| 5490a7e9eba55b4b05a1c615 | 2    | core | {u'url': u'2'} | core.st2.webhook |
+--------------------------+------+------+----------------+------------------+
```
Alternate formats -
```
$ st2 trigger getspecs core.st2.webhook -j
[
    {
        "id": "5490a7dceba55b4b05a1c614",
        "type": "core.st2.webhook",
        "name": "1",
        "parameters": {
            "url": "1"
        },
        "pack": "core"
    },
    {
        "id": "5490a7e9eba55b4b05a1c615",
        "type": "core.st2.webhook",
        "name": "2",
        "parameters": {
            "url": "2"
        },
        "pack": "core"
    }
]
```