Merge branch 'stable-2.15'

* stable-2.15:
  ElasticRestClientProvider: Make client field volatile
  AbstractElasticIndexer: Specify charset for InputStreamReader
  SshCommandsIT: Use immutable type for constant field declarations
  rest-api-config.txt: Fix monospace `plugin`

Change-Id: Ib03a5976e13887bae3bf44f0864459418ed8a4f5