Strip trailing forward slashes from elasticsearch URLs

What does this MR do?

Converts URLs like http://localhost:9200/ to http://localhost:9200

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

Why was this MR needed?

URLs with trailing slashes are not supported by elasticsearc-ruby: https://github.com/elastic/elasticsearch-ruby/issues/278

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 #2039 (closed)