A zone import defaults to a ttl of zero (outside schema limits)

To reproduce, create a zone with no ttls specified:

    POST /v2/zones/tasks/imports HTTP/1.1
    Content-Type: text/dns
    Accept: application/json

    $ORIGIN wumbo.com.
    wumbo.com. IN SOA ns.wumbo.com. nsadmin.wumbo.com. 101 102 103 104 105
    wumbo.com. IN NS ns.wumbo.com.
    ns.wumbo.com. IN A 127.0.0.1

Designate assigns a ttl of zero to the zone:

    {
      "status": "ACTIVE",
      "name": "wumbo.com.",
      "id": "447c28b7-f117-4366-b017-11afc1bef7d9",
      "ttl": 0,
      ...
    }

This is outside of the allowed ttl boundaries, according to the schema: https://github.com/openstack/designate/blob/master/designate/objects/zone.py#L57-L63.

This is a problem because all updates to the zone will fail (provided the update does not rectify the ttl). For example,

Request:

    ## Patching the zone with an empty json object
    PATCH /v2/zones/447c28b7-f117-4366-b017-11afc1bef7d9 HTTP/1.1
    Content-Type: application/json
    Accept: application/json

    {}

Response:

    {
      "code": 400,
      "type": "invalid_object",
      "errors": {
        "errors": [
          {
            "path": [
              "ttl"
            ],
            "message": "0 is less than the minimum of 1",
            "validator": "minimum",
            "validator_value": 1
          }
        ]
      },
      "request_id": "req-7055407e-87b0-45c0-8540-8ed4101f066a"
    }