[Buildkite] Enable independent Elastic Agents and for some packages enable parallel system tests

[mrodm]

Relates elastic/elastic-package#787

Enable independent Elastic Agents for all packages, it will require to update auditd_manager and oracle packages. It needs to define the environment variable ELASTIC_PACKAGE_TEST_ENABLE_INDEPENDENT_AGENT=true

• Related to the changes in Enable and update packages using agent service deployer - DO NOT MERGE #9862

Enable system tests in parallel for network_traffic and zeek packages:

• Requires to add the configuration file _dev/test/config.yml
• Requires to set the maximum number of routines with the environment variable ELASTIC_PACKAGE_MAXIMUM_NUMBER_PARALLEL_TESTS=5
• Related to the changes in Test parallel tests - DO NOT MERGE #10161

Checklist

• Requires a new elastic-package version released and updated in this repository

EDIT: Move auditd_manager and oracle packages update to another issue.

[mrodm]

Updated description issue to just enable independent Elastic Agents and enable running system tests in parallel for network_traffic and zeek packages.

Updating auditd_manager and oracle packages is going to be done in a second stage, once elastic-package is running independent Elastic Agents by default. A new elastic-package release containing elastic/elastic-package#1954 is required.

Created issue for this second stage: #10390