[dogshell] Enforce the default 'normal' event priority client side

[brouberol]

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What does this PR do?

Enforce the default event priority value to normal when submitting an event via the API.

Description of the Change

The CLI help output stats that the default priority value used when
posting an event with dog event post is normal. That value isn't
enforced on the client side, and it seems leaving it to None has an
unexpected side effect: the API returns an HTTP 200 response along with
an link to the indexed event, which always returns an HTTP 404.

Example:

$ dog --raw event post --alert_type 'info' --no_host 'test by balthazar' 'checking for any pebcak' | jq .
{
  "status": "ok",
  "event": {
    "id": XXXX,
    "title": "test by balthazar",
    "text": "checking for any pebcak",
    "date_happened": 1578645516,
    "handle": null,
    "priority": null,
    "related_event_id": null,
    "tags": [],
    "url": "https://app.datadoghq.com/event/event?id=1234"
  }
}

That event url always returns an HTTP 404 when opened. We can see
"priority": null in the response payload.

If we however explicitly set the --priority flag to normal in the
command-line, we can see "priority": "normal" in the JSON output:

$ dog --raw event post --priority normal --alert_type 'info' --no_host 'test by balthazar' 'checking for any pebcak' | jq .
{
  "status": "ok",
  "event": {
    "id": YYYYY,
    "title": "test by balthazar",
    "text": "checking for any pebcak",
    "date_happened": 1578645659,
    "handle": null,
    "priority": "normal",
    "related_event_id": null,
    "tags": [],
    "url": "https://app.datadoghq.com/event/event?id=ABCD"
  }
}

Visiting the event URL now returns an HTTP 200, and I can find the event
by searching in the event stream.

Additional API validation will of course have to be added server-side
but this is at least a start.

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.

[gzussa]

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