elasticsearch: add 7.x version support (#16646)

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

public/app/plugins/datasource/elasticsearch/elastic_response.ts

@@ -184,7 +184,7 @@ export class ElasticResponse {
   }
 
   // This is quite complex
-  // need to recurise down the nested buckets to build series
+  // need to recurse down the nested buckets to build series
   processBuckets(aggs, target, seriesList, table, props, depth) {
     let bucket, aggDef, esAgg, aggId;
     const maxDepth = target.bucketAggs.length - 1;
@@ -310,11 +310,13 @@ export class ElasticResponse {
   }
 
   processHits(hits, seriesList) {
+    const hitsTotal = typeof hits.total === 'number' ? hits.total : hits.total.value; // <- Works with Elasticsearch 7.0+
+
     const series = {
       target: 'docs',
       type: 'docs',
       datapoints: [],
-      total: hits.total,
+      total: hitsTotal,
       filterable: true,
     };
     let propName, hit, doc, i;