Cross Region Searching

It was agreed at the Mitaka summit to restrict this to adding region fields to documents to allow clients to potentially aggregate results, and not tackle cross-region deployment for now.

SEE: https://review.openstack.org/#/c/301227/

Summary
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Provide cross region searching.

Motivation
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A primary ask for Searchlight is the ability to search across resources, across regions, across projects. This is the across regions portion of that ask.

Description
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Searchlight is currently not indexing region based data. We have to explore the queries and deployments required to support cross region searching.

Testing
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Brief instruction for reviewers to exercise the changes, including expected results where non-obvious.

Outside Dependencies
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Is this functionality already supported in other services? List the appropriate API calls and if they are extensions or base API functionality.

This should describe any cross project dependencies. This should include:
  * changes to OpenStack services
  * changes to OpenStack services clients (python-*client)
  * changes in external projects
Links to particular patches should be included in the whiteboard area.

Requirements Update Required
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