add support for instance profile credentials when using AWS hosted elasticsearch

What does this MR do?

Adds support for instance profile credentials when used with AWS hosted elasticsearch

Are there points in the code the reviewer needs to double check?

/lib/gitlab/elastic/client.rb

Why was this MR needed?

Elasticsearch AWS support does not work with instance role credentials

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2169 (closed)