[ITs] change healthcheck for elasticsearch

[v1v]

What does this PR do?

Elasticsearch docker image is based on centos:8 but it doesn't have python installed.

Why is it important?

Healthcheck with the python approach are not supported anymore since python is not installed. curl is the less intrusive approach instead.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

## How to test this

gvm install go1.14.4
gvm use go1.14.4
make mage
cd x-pack/libbeat
mage build test

...
-------------------------------------------------- generated xml file: /go/src/github.com/elastic/beats/x-pack/libbeat/build/TEST-python-integration.xml ---------------------------------------------------
=========================================================================================== slowest 20 durations ===========================================================================================
14.83s call     x-pack/libbeat/tests/system/test_management.py::TestManagement::test_fetch_configs
12.87s call     x-pack/libbeat/tests/system/test_management.py::TestManagement::test_configs_cache
1.94s call     x-pack/libbeat/tests/system/test_management.py::TestManagement::test_enroll
0.42s call     x-pack/libbeat/tests/system/test_management.py::TestManagement::test_enroll_bad_pw

(8 durations < 0.005s hidden.  Use -vv to show these durations.)
============================================================================================ 4 passed in 33.36s ============================================================================================
>> python test: Integration Testing Complete

Logs

docker inspect $(docker ps | grep docker.elastic.co/elasticsearch/elasticsearch:8.0.0-SNAPSHOT | cut -f1 -d' ') --format "{{json .State.Health }}" | jq

{
  "Status": "starting",
  "FailingStreak": 752,
  "Log": [
    {
      "Start": "2020-08-10T08:00:34.210390517Z",
      "End": "2020-08-10T08:00:34.315594515Z",
      "ExitCode": 127,
      "Output": "/bin/sh: python: command not found\n"
    },

Issues

Duplicates #20511

[v1v]

Same approach as done in some other projects:

• https://github.com/elastic/opbeans-java/blob/master/docker-compose.yml#L91

[jsoriano]

Thanks!

[v1v]

Unrelated test failures