[ML] Estimate bucket span not working when search.max_buckets set in dynamic cluster settings

[sw-jung]

Kibana version: 7.4.x, 7.5.1

Elasticsearch version: 7.4.x, 7.5.1

Server OS version: Cloud

Browser version: Chromium 80

Browser OS version: Windows 10

Original install method (e.g. download page, yum, from source, etc.): Cloud

Describe the bug: Estimate bucket span not working when search.max_buckets set in dynamic cluster settings with message Unable to retrieve cluster setting search.max_buckets.

Steps to reproduce:

1. Set search.max_buckets in dynamic cluster settings like below:

PUT _cluster/settings
{
  "transient": {
    "search.max_buckets": 20000
  }
}

2. Go to ML page
3. Create a random new single metric job.
4. Click the estimate bucket span button.

Expected behavior: Estimate bucket span button should be working even if search.max_buckets set in cluster settings.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/ml-ui (:ml)