[1181] Expose low level elasticsearch client params

  * replace elasticsearch_host and elasticsearch_port with elasticsearch_url
  * Add support for AWS Elasticsearch Service
    * created universal gitlab elasticsearch client
    * add ability to sign requests with aws_signers_v4
    * expose elasticsearch_aws_region param
    * expose elasticsearch_aws_access_key param
    * expose elasticsearch_aws_secret_access_key param
    * If using AWS instance credentials they will automatically be picked up by client