Fix stuck when executing a background command via SSH is SA02

After upgrading the image to Ubuntu 20 in hackfest_basic_metrics_vnf
the SA02 test started to fail: the "Execute Command" keyword from
SSHLibrary hangs when running a "yes" command in backgroud in the VDU
and test never ends. It seems that the SSH library does not handle
the execution of background (run using '&') processes well.

Change-Id: I66aed317ae12cb117ddd1a3cdec3396185dfdbe1
Signed-off-by: aguilard <e.dah.tid@telefonica.com>