Disable ILM on ELK vendor yaml

What does this MR do?

TLDR; ILM is enabled by default (with non-daily indices) in ELK 7.6.2 and WAF statistics is expecting daily indices.

There are more details and a test case in this issue: #222953 (closed)

With this change we will have control over the index in a way that this logic can be simplified in a follow-up MR.

I have tested this solution (Fri Jun 26 02:05:02 UTC 2020) on ELK deployed on VMs and there is a current test running on GKE cluster with the following status:

[elasticsearch@elastic-stack-elasticsearch-master-0 ~]$ date
Fri Jun 26 02:05:02 UTC 2020
[elasticsearch@elastic-stack-elasticsearch-master-0 ~]$ curl http://localhost:9200/_cat/indices
green open filebeat-7.6.2-2020.06.26 Frf5UZv0S9OX9_GJuMA4pg 1 1 1598 0 2.5mb 1.2mb
[elasticsearch@elastic-stack-elasticsearch-master-0 ~]$ curl http://localhost:9200/filebeat-7.6.2-2020.06.26/_count
{"count":1775,"_shards":{"total":1,"successful":1,"skipped":0,"failed":0}}

The following test was performed on Jan 27 UTC 2020 and it seems that the daily index is working once again:

[elasticsearch@elastic-stack-elasticsearch-master-0 ~]$ date
Sat Jun 27 00:10:18 UTC 2020
[elasticsearch@elastic-stack-elasticsearch-master-0 ~]$ curl http://localhost:9200/_cat/indices
green open filebeat-7.6.2-2020.06.26 Frf5UZv0S9OX9_GJuMA4pg 1 1 109322 0 169.8mb 84.8mb
green open filebeat-7.6.2-2020.06.27 FtPw-aySRzuVdZ8G3fQCFg 1 1    884 0   1.9mb    1mb
[elasticsearch@elastic-stack-elasticsearch-master-0 ~]$ curl http://localhost:9200/filebeat-7.6.2-2020.06.26/_count
{"count":109322,"_shards":{"total":1,"successful":1,"skipped":0,"failed":0}}
[elasticsearch@elastic-stack-elasticsearch-master-0 ~]$ curl http://localhost:9200/filebeat-7.6.2-2020.06.27/_count
{"count":985,"_shards":{"total":1,"successful":1,"skipped":0,"failed":0}}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team