Add admin setting of Elasticsearch client request timeout

What does this MR do?

This MR is to expose the Elasticsearch client request timeout setting to admin user.

Indexer side of change is in gitlab-elasticsearch-indexer!91 (merged)

Migration output

# bundle exec rails db:migrate RAILS_ENV=development
== 20200903054946 AddElasticsearchClientTimeout: migrating ====================
-- add_column(:application_settings, :elasticsearch_client_request_timeout, :integer, {:null=>false, :default=>0})
   -> 0.0100s
== 20200903054946 AddElasticsearchClientTimeout: migrated (0.0101s) ===========

# bundle exec rails db:migrate:down VERSION=20200903054946 RAILS_ENV=development
== 20200903054946 AddElasticsearchClientTimeout: reverting ====================
-- remove_column(:application_settings, :elasticsearch_client_request_timeout, :integer, {:null=>false, :default=>0})
   -> 0.0130s
== 20200903054946 AddElasticsearchClientTimeout: reverted (0.0163s) ===========

Screenshots

In the screenshot below, the `Client request timeout is the newly added config.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #224526 (closed)

Closes #224526 (closed)