json matches an md5 hash. This was causing flaky tests because the
order of the users in the test doing `curl -s localhost:9200/_xpack/security/user -u
es_admin:changeMeAgain` is not consistent which caused random failures.
Now if it does fail we will actually know the reason why:
```
security users
should have the elastic user
should have the kibana user (FAILED - 1)
should have the kibana_server user
should have the logstash user
logstash_system access check
should be reported as version 6.2.4
Failures:
1) Xpack Tests security users should have the kibana user
Failure/Error: expect(result['kibana']['username']).to eq('kibana4')
expected: "kibana4"
got: "kibana"
```
The failure output use to look like:
```
Failures:
1) Xpack Tests Command "curl -s localhost:9200/_xpack/security/user -u es_admin:changeMeAgain | md5sum | grep b6a1293c343e745a508c74778c9be8bb" exit_status should eq 0
Failure/Error: its(:exit_status) { should eq 0 }
expected: 0
got: 1
(compared using ==)
/bin/sh -c curl\ -s\ localhost:9200/_xpack/security/user\ -u\ es_admin:changeMeAgain\ \|\ md5sum\ \|\ grep\ b6a1293c343e745a508c74778c9be8bb
```
Renamed vars, set the tasks to use the es_user and es_group vars in elasticsearch-optional-user.yml. Modifed README.md to list the es_user_id and es_group_id vars, made note that both vars are required if they are used. Removed the commented out instances of the vars in defaults/main.yml.
Added es_user, es_group, es_user_id, es_group_id vars to config testing.
Made conditionals use explicit 'is defined' test.
Add tests for es_user_id and es_group_id.
Update defaults/main.yml and test/integration/config.yml to match master plus test config additions.
