Cherry-pick #23831 to 7.x: Add system.hostfs configuration option for system module

[ChrsMark]

Cherry-pick of PR #23831 to 7.x branch. Original message:

What does this PR do?

This PR adds system.hostfs configuration option for system module.
It will be used instead of the flag -system.hostfs=/hostfs where adding a flag is not possible ie in Agent. Using the -system.hostfs will still be available until we completely remove it in 8.0

Why is it important?

In some cases setting the proc filesystem using the -system.hostfs is not possible like when running with Agent, see #22915. A configuration option should be used instead.

How to test this PR manually

1. Build Metricbeat: GOOS=linux GOARCH=amd64 go build
2. Prepare a configuration file vim system.yml:

- module: system
  period: 10s
  metricsets:
    - process
  # Configure the mount point of the host’s filesystem for use in monitoring a host from within a container
  system.hostfs: "/hostfs"

3. Use the following Dockerfile.debug to build a custom image locally:

FROM docker.elastic.co/beats/metricbeat:7.10.2
COPY metricbeat.yml /usr/share/metricbeat/metricbeat.yml
COPY metricbeat /usr/share/metricbeat/metricbeat
COPY system.yml /usr/share/metricbeat/modules.d/system.yml
USER root
RUN chown root:metricbeat /usr/share/metricbeat/metricbeat.yml
USER metricbeat

4. Build the image: docker build -f Dockefile.debug . -t metricbeat-hostfs
5. Run Metricbeat and verify that process metricset collects data:

docker run --mount type=bind,source=/proc,target=/hostfs/proc,readonly --mount type=bind,source=/sys/fs/cgroup,target=/hostfs/sys/fs/cgroup,readonly --mount type=bind,source=/,target=/hostfs,readonly --net=host metricbeat-hostfs -e -d "*" 

6. Try to break Metricbeat by adding a random hostpath flag: -system.hostfs=/hostfs45

Related issues

• Closes Add Metricbeat --system.hostfs equivalent to Elastic Agent #22915
• Relates [Elastic Agent] [Meta] Gathering logs and metrics from Kubernetes without using fleet generated agent config and without central agent management via fleet #23613

cc: @blakerouse

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #23900 updated

• • Start Time: 2021-02-08T08:25:29.060+0000

• Duration: 34 min 43 sec

• Commit: ad33c5b

Test stats 🧪

Test	Results
Failed	0
Passed	8694
Skipped	2140
Total	10834

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	8694
Skipped	2140
Total	10834

[mtojek]

LGTM