[CI] disable aws cloud testing by default

[v1v]

What does this PR do?

Run Cloud Testing on demand using the UI, comments or labels or on a weekly basis for the master and 7.x branches.

Triggers:

• UI based with the parameter awsCloudTests
• GH comment /test x-pack/metricbeat for aws cloud
• GH Label aws

Why is it important?

Cloud Testing got a limitation with the number of db instances concurrently with the current service account used in the CI

[2020-12-14T15:41:25.769Z] Error: Error creating DB Instance: InstanceQuotaExceeded: DB Instance quota exceeded
[2020-12-14T15:41:25.769Z] 	status code: 400, request id: 7ce18407-a537-4d4e-ad21-75cd3730160b
[2020-12-14T15:41:25.769Z] 
[2020-12-14T15:41:25.769Z]   on terraform.tf line 18, in resource "aws_db_instance" "test":
[2020-12-14T15:41:25.769Z]   18: resource "aws_db_instance" "test" {

The quotas and limitations can be found in the limits section.

Issues

See #23105 (comment)

cc @jsoriano and @ph as you commented in #23105 (comment)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #23186 updated

• Start Time: 2021-01-06T18:39:37.300+0000

• Duration: 53 min 26 sec

Test stats 🧪

Test	Results
Failed	0
Passed	17805
Skipped	1385
Total	19190

Steps errors

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Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 4 sec . View more details on here
• Description: terraform apply -auto-approve

Terraform Apply on x-pack/metricbeat/module/aws

• Took 0 min 4 sec . View more details on here
• Description: terraform apply -auto-approve

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	17805
Skipped	1385
Total	19190