Define matrix compatibility for metricbeat and the elastic stack outside the test framework

[mdelapenya]

We want the automation tool to define the different combination of input parameters to generate different scenarios:

Stack 7.4 + metricbeat 7.4 + tests for apache (2.2, 2.4)
Stack 7.5-SNAPSHOT + metricbeat 7.5-SNAPSHOT + tests for apache (2.2, 2.4)
Stack 7.5-SNAPSHOT + metricbeat 7.4 + tests for apache (2.2, 2.4)

The test framework could read stack and metricbeat versions from the environment (config test file, environment variable, etc), keeping just the versions of the integration module in the feature file.

For local development, we must ensure a default value for those two arguments too.

cc/ @elastic/observablt-robots

[mdelapenya]

Closing as it's implemented, and the CI is able to trigger different jobs with multiple combinations