Fix advanced-reboot.py not detecting the retry count option in warm-reboot (sonic-net#11676)

On images where the warm-reboot (and fast-reboot) script supports the
`-n` and `-N` flags, which either require or not require the teamd retry
count feature to be enabled, the advanced-reboot.py script is supposed
to check to see if this feature is present, and then use `-n` or `-N`
depending on if SONiC neighbors are being used or not. However, this
never appeared to be being used. The reason for this is that `stdout` is
actually an array of strings, so using `in` just meant if the test
string exactly matched some line from the help output, which will never
be true here.

The quick and dirty way to fix this is to concatenate this into a single
string (newline-separated), and then use `in`.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

Author: saiarcot895

ansible/roles/test/files/ptftests/py3/advanced-reboot.py

@@ -1475,7 +1475,7 @@ def reboot_dut(self):
             # Check to see if the warm-reboot script knows about the retry count feature
             stdout, stderr, return_code = self.dut_connection.execCommand(
                 "sudo " + self.reboot_type + " -h", timeout=5)
-            if "retry count" in stdout:
+            if "retry count" in "\n".join(stdout):
                 if self.test_params['neighbor_type'] == "sonic":
                     reboot_command = self.reboot_type + " -N"
                 else: