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>
Merge pull request #1512 from ypu/net_env_debug

virttest: Add debug option to ssh and dhclient command