ffdc: Trigger Paramiko exec_command() timeout

Read stderr instead of stdout to trigger Paramiko exec_command() timout mechanism.
This fix timeing issue with previous patch.

Test:
- Regression test to openbmc, ubuntu, rhel with standard config and plugins

Signed-off-by: Peter D  Phan <peterp@us.ibm.com>
Change-Id: I793ba98e638fcdf8214e491760d8a94af714a41a
diff --git a/ffdc/lib/ssh_utility.py b/ffdc/lib/ssh_utility.py
index ea5cf4f..1ca1b05 100644
--- a/ffdc/lib/ssh_utility.py
+++ b/ffdc/lib/ssh_utility.py
@@ -92,19 +92,19 @@
             while time.time() < start + default_timeout:
                 # Need to do read/write operation to trigger
                 # paramiko exec_command timeout mechanism.
-                xresults = stdout.readlines()
+                xresults = stderr.readlines()
                 results = ''.join(xresults)
+                time.sleep(1)
                 if stdout.channel.exit_status_ready():
                     break
-                time.sleep(1)
             cmd_exit_code = stdout.channel.recv_exit_status()
 
             # Convert list of string to one string
             err = ''
             out = ''
-            for item in stderr.readlines():
-                err += item
             for item in results:
+                err += item
+            for item in stdout.readlines():
                 out += item
 
             return cmd_exit_code, err, out