Fix to use non-nested parameters for datadog_event operator

[NomadBlacky]

Typical operators are using non-nested parameters.
But datadog_event operator is using nested parameters.

+simple_query_nonexpanded:
  td>:
  # non-nested
  query: "SELECT * FROM nasdaq"

+example:
  datadog_event>:
    # nested
    title: "[TEST] digdag-plugin-datadog"
    text: "Digdag meets Datadog!!"
    tags:
      - "env:test"
    alert_type: info # is default. success, warning or error
    priority: normal # is default. or low

I think should use non-nested parameters making no misleading.
However, I prefer nested parameters because logs are more clear.

Before

digdag run --no-save --param home=$HOME events.dig
2020-03-07 18:01:59 +0900: Digdag v0.9.41
2020-03-07 18:02:00 +0900 [WARN] (main): Using a new session time 2020-03-07T00:00:00+00:00.
2020-03-07 18:02:00 +0900 [INFO] (main): Starting a new session project id=1 workflow name=events session_time=2020-03-07T00:00:00+00:00
2020-03-07 18:02:01 +0900 [INFO] (0016@[0:default]+events+example): datadog_event>: {title=[TEST] digdag-plugin-datadog, text=Digdag meets Datadog!!, tags=[env:test], alert_type=info, priority=normal}
2020-03-07 18:02:05 +0900 [INFO] (0016@[0:default]+events+example): Start the datadog_event operation.
2020-03-07 18:02:06 +0900 [INFO] (0016@[0:default]+events+example): Succeeded to post the event to Datadog. https://app.datadoghq.com/event/event?id=5357113725809002349
Success.

After

$ digdag run --no-save --param home=$HOME events.dig
2020-03-07 18:03:50 +0900: Digdag v0.9.41
2020-03-07 18:03:54 +0900 [WARN] (main): Using a new session time 2020-03-07T00:00:00+00:00.
2020-03-07 18:03:54 +0900 [INFO] (main): Starting a new session project id=1 workflow name=events session_time=2020-03-07T00:00:00+00:00
2020-03-07 18:03:55 +0900 [INFO] (0016@[0:default]+events+example): datadog_event>:
2020-03-07 18:03:57 +0900 [INFO] (0016@[0:default]+events+example): Start the datadog_event operation.
2020-03-07 18:04:00 +0900 [INFO] (0016@[0:default]+events+example): Succeeded to post the event to Datadog. https://app.datadoghq.com/event/event?id=5357115636173312813
Success.