Adds a new version option 7.0+ (70 internally).
Version 7.0+ doesn't include document types in index mappings
so support for handling this have been added.
Version 7.0+ returns number of hits in a different way so
support for handling this have been added.
Version 7.0+ doesn't support sending
max_concurrent_shard_requests in multisearch header so
support for sending this in query string have been added.
Update elastic6 docker block and dashboards (devenv) to use
6.7.1 images, filebeat index name is now filebeat-YYYY.MM.DD
and dashboard include correct tags and links.
Add elastic7 docker block and provisioning (devenv).
Updates documentation regarding new version.
Closes#15622
