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?

What are the relevant issue numbers?

Closes #2169 (closed)