[system-health] Make run_command() Python 3-compliant (#6371)

Pass universal_newlines=True parameter to subprocess.Popen(); no longer use .encode('utf-8') on resulting stdout.
This was missed in #5886

Note: I would prefer to use text=True instead of universal_newlines=True, as the former is an alias only available in Python 3 and is more understandable than the latter. However, Even though the setup.py file for this package only specifies Python 3, the LGTM tool finds other Python 2 code in the repo and validates the code as Python 2 code and alerts that text=True is an invalid parameter. Will stick with universal_newlines=True for now. Once all Python code in the repo has been converted to Python 3, I will change all universal_newlines=True to text=True.

src/system-health/health_checker/utils.py

@@ -8,8 +8,8 @@ def run_command(command):
     :return: Output of the shell command.
     """
     try:
-        process = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE)
-        return process.communicate()[0].encode('utf-8')
+        process = subprocess.Popen(command, shell=True, universal_newlines=True, stdout=subprocess.PIPE)
+        return process.communicate()[0]
     except Exception:
         return None