Make the number of ES shards and replicas configurable

What does this MR do?

Adds a configuration setting for the number of elasticsearch shards and replicas an index is created with. This affects the operation of the gitlab:elastic:create_empty_index rake task only.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #2087 (closed)