Pass logging.level from elastic agent to processes.

[michel-laterman]

What does this PR do?

Insert the logging.level config setting into the configuration passed to
sub processes when emitting a config from a policy.

Why is it important?

Allows operators to view debug logs for the beats/processes which are started by the agent by altering the log level.
The agent's log level value will be passed to all processes it creates.
This can already be done today by altering it in Kibana

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

1. install/enroll elastic agent to fleet
2. change log level of agent in fleet
3. view agent logs and metricbeat/filebeat logs

Related issues

• Closes Fleet managed: enable debug mode  elastic-agent#143

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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Build stats

• Start Time: 2021-10-28T22:27:09.928+0000

• Duration: 90 min 3 sec

• Commit: d28b770

Test stats 🧪

Test	Results
Failed	0
Passed	7120
Skipped	16
Total	7136

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[michel-laterman]

As commented on the issue we would be better specifying the level in the policy (that can be read from the env, with a default)