[FA] Stop relying on hardcoded value of run_path

[coignetp]

What does this PR do?

Rely on the datadog_agent config run_path instead of the hardcoded /opt/datadog-agent

Motivation

If a user installs the agent with Remote Management enabled, the installation path of the agent is /opt/datadog-packages/datadog-agent/<version>, so the default value was not working

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

Attention: Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.05%. Comparing base (1a00621) to head (a9b26b5).
Report is 16 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
confluent_platform	?
glusterfs	78.96% <45.45%> (-1.04%)	⬇️
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
presto	?
solr	?
tomcat	?
weblogic	?

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