Fix osutil/default route_add to pass string array.

[gram-signal]

Description

In common/dhcp.py::DhcpHandler.conf_routes, a default gateway is added via:

if self.gateway is not None and self.osutil.is_missing_default_route():
  self.osutil.route_add(0, 0, self.gateway)

In osutil/default::DefaultOSUtil.route_add, this creates a command and runs it with:

cmd = ["ip", "route", "add", net, "via", gateway]
return shellutil.run_command(cmd)

Since net is the integer 0, this results in the following issue:

  File "/usr/lib/python3/dist-packages/azurelinuxagent/common/dhcp.py", line 132, in conf_routes
    self.osutil.route_add(0, 0, self.gateway)
  File "/usr/lib/python3/dist-packages/azurelinuxagent/common/osutil/default.py", line 1138, in route_add
    return shellutil.run_command(cmd)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/azurelinuxagent/common/utils/shellutil.py", line 255, in run_command
    return __run_command(command_action=command_action, command=command, log_error=log_error, encode_output=encode_output)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/azurelinuxagent/common/utils/shellutil.py", line 182, in __run_command
    return_code, stdout, stderr = command_action()
                                  ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/azurelinuxagent/common/utils/shellutil.py", line 245, in command_action
    process = _popen(command, stdin=popen_stdin, stdout=stdout, stderr=stderr, shell=False)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/azurelinuxagent/common/utils/shellutil.py", line 352, in _popen
    process = subprocess.Popen(*args, **kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 1024, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.11/subprocess.py", line 1834, in _execute_child
    self.pid = _fork_exec(
               ^^^^^^^^^^^
TypeError: expected str, bytes or os.PathLike object, not int

This fixes the issue by converting net to a string.

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• New Unit tests were added for the changes made

Quality of Code and Contribution Guidelines

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[narrieta]

@gram-signal Thank you for your contribution, this has been merged in #3072