Add a cleaner solution to store global entities in elasticsearch.

[bricht]

As a part of #357 variant entities were added to elastic storage.
These entities are store-scope independent, and thus did not fit the existing creation and naming pattern of indexes.

A temporary solution was added, but this should be relaced with a cleaner one and tested for products, categories and variants.

See: Magento\StorefrontStorage\Model\State::getCurrentDataSourceName

[m2-assistant]

Hi @bricht. Thank you for your report.
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[mslabko]

@magento export issue to Jira project SFAPP as Story

[github-jira-sync-bot]

✅ Jira issue (https://jira.corp.magento.com/browse/SFAPP-314) is successfully created for this issue.