Fix settings for newly created incides

What does this MR do and why?

It turns out that we were merging all index settings, which caused this bug: gitlab-production index gets created with a sha... (#420347 - closed)

In this MR we update the elastic helper to use correct settings.

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Before	After

How to set up and validate locally

1. Enable Advanced Search
2. Checkout master
3. Update number of shards for gitlab-development, but keep it different from all other indices (for example, 10)
4. Execute bin/rake gitlab:elastic:recreate_index
5. Ensure that curl -s http://localhost:9200/gitlab-development/_settings | jq . | fgrep number_of_shards returns not the correct number of shards
6. Checkout this MR's branch
7. Execute bin/rake gitlab:elastic:recreate_index
8. Ensure that curl -s http://localhost:9200/gitlab-development/_settings | jq . | fgrep number_of_shards returns the correct number of shards

MR acceptance checklist

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Related to #420347 (closed)