Fix metadata user data parse

[superzc-github]

What does this PR do?

This is to make sure user-data in grains metadata will be returned correctly

What issues does this PR fix or reference?

N/A

Previous Behavior

user-data content will be cut by this current condition:
elif "=" in line:
key, value = line.split("=")
ret[value] = _search(prefix=os.path.join(prefix, key))

New Behavior

user-data will be return with its whole content directly with any further cut/parse for it's content

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

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Yes/No

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[github-actions]

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Detected Issues (click me)

Check Known Missing Docstrings...........................................Failed
- hook id: invoke
- duration: 1.13s
- exit code: 1
/home/runner/.cache/pre-commit/repoyt5u40n1/py_env-python3/lib/python3.9/site-packages/_distutils_hack/init.py:30: UserWarning: Setuptools is replacing distutils.

warnings.warn("Setuptools is replacing distutils.")

The function 'metadata' on 'salt/grains/metadata.py' does not have a docstring

Found 1 errors

---

Thanks again!

[dwoz]

@superzc-github Are you able to add a test case for this change?