From Chef Server 13.2.0, we are capable of using version 6 of ElasticSearch along with 2 and 5.

Tested the stack with ES6, working OK and also passes chef-server-ctl test

Also enabled in-place upgrade of the ElasticSearch domain by adding

  ElasticsearchDomain:
    Type: AWS::Elasticsearch::Domain
    UpdatePolicy:
      EnableVersionUpgrade: true

• https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/es-version-migration.html
• https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-updatepolicy.html#cfn-attributes-updatepolicy-upgradeelasticsearchdomain

Added "Upgrading ElasticSearch" section to the README to describe process of upgrading Chef Server to 13.2.0, followed by an upgrade to ElasticSearch 6. Tested by:

1. Deploying the 5.1.1 version of the stack with ES5.6, uploading policies and bootstrapping some nodes.
2. Deployed a change set with template version 5.2.0 to upgrade to Chef Server 13.2.0
3. Deployed a further change set to upgrade to ES6.8.
4. Verified nodes and searching all working OK and passes chef-server-ctl test too. There was some downtime whilst the ES5 to 6 upgrade was being processed by AWS.

This PR will also resolve #76 as AMIs also include latest Automate 20200513205422 which includes this change from 20200506172513:

Quick Change: Adding a legacy data collector token automatically adds the created API token to the IAM Ingest Policy.