redirect ssh debug message to /dev/null

The following commit added a '-vvv' option to ssh, then ssh will output
some debug message. If the debug message (like "debug2: channel 0:
window 999411 sent adjust 49165") mixes with the command output, it will
confuse Autotest aexpect, exceptions are raised in executing command.

This patch redirect the debug message to /dev/null, if the debug message
is needed, please save it to a file by another patch.

| commit b596354972eecafdcc493577eeaf9d3d4e51c744
| Author: Yiqiao Pu <ypu@redhat.com>
| Date:   Fri Mar 14 15:33:27 2014 +0800
|
|     virttest: Add debug option to ssh and dhclient command
|
|     Add debug option to ssh and dhclient command to get more information
|     to figure out if a test failed is caused by netowrk env problem.
|     Also let dhclient do not run in background to show the infomrmation.
|
|     Signed-off-by: Yiqiao Pu <ypu@redhat.com>

Signed-off-by: Amos Kong <akong@redhat.com>