Add system level logs for Agent on k8s

[ChrsMark]

What does this PR do?

This PR adds system integration to collect system level logs from k8s nodes:

- var/log/auth.log*
- /var/log/secure*
- /var/log/messages*
- /var/log/syslog*

Tested on GKE (ubuntu based nodes) but manual installation of rsyslog is required since it's not there by default:
sudo apt-get -y install rsyslog && sudo service rsyslog start

Why is it important?

So as to collect system level logs from k8s nodes on which Agent runs as Pod.

Related issues

• Relates [Elastic Agent] [Meta] Gathering logs and metrics from Kubernetes without using fleet generated agent config and without central agent management via fleet #23613

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Pinging @elastic/integrations (Team:Integrations)

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Build stats

• Build Cause: Pull request #24185 updated

• Start Time: 2021-02-23T12:42:18.337+0000

• Duration: 54 min 5 sec

• Commit: f22b407

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[blakerouse]

Looks great!