Add elastic test rake task

What does this MR do?

In order to make testing elasticsearch changes a bit easier, this MR adds a few raketasks:

• gitlab:elastic:test:index_size: Returns document count and index size for currently-used index
• gitlab:elastic:test:index_size_change: Runs the index_size task, reindexes, and runs the task again
• gitlab:elastic:test:setup_repos[fork]: Imports some large popular opensource repos to have more blobs to test with. If fork is provided, it's value should be either "true" or the full_path of a project
  • if the value is "true" it will generate five forks of torvalds/linux
  • otherwise it will generate five forks of the given project

What are the relevant issue numbers?

#7822 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7822 (closed)