Add json formatted information to / request and deprecate healthcheck

[simitt]

Add information about server version etc. in json format to the root endpoint, and deprecate the healthcheck endpoint.

[alvarolobato]

We should limit the provided information if you don't have the secret token.
We should also expose libbeat information.

[graphaelli]

Current Elastic implementations:

elasticsearch

{
  "cluster_name": "docker-cluster",
  "cluster_uuid": "sam16O2yR8uCnuOkd_s8Eg",
  "name": "nVvo3JI",
  "tagline": "You Know, for Search",
  "version": {
    "build_date": "2018-07-04T17:01:27.342712Z",
    "build_flavor": "default",
    "build_hash": "c067c02",
    "build_snapshot": true,
    "build_type": "tar",
    "lucene_version": "7.3.1",
    "minimum_index_compatibility_version": "5.0.0",
    "minimum_wire_compatibility_version": "5.6.0",
    "number": "6.3.1"
  }
}

beats (http.enabled=true):

{
  "beat": "apm-server",
  "hostname": "foo.local",
  "name": "foo.local",
  "uuid": "617cec24-0ad4-451e-8074-fd138baff41d",
  "version": "7.0.0-alpha1"
}

logstash

{
  "build_date": "2018-07-04T18:01:08Z",
  "build_sha": "cb062c5699b1a8ff6eb7029072d514c9fd8e099d",
  "build_snapshot": true,
  "host": "ca81d341dca9",
  "http_address": "0.0.0.0:9600",
  "id": "537a913b-2ee9-4fd3-9af0-d9da285fae22",
  "name": "ca81d341dca9",
  "version": "6.3.1"
}