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: Ibe37b0b53c8196bb555b7d7d5f6da1365f24326d
Signed-off-by: Ubuntu <e.dah.tid@telefonica.com>
(cherry picked from commit f664aa79)

robot-systest/testsuite/sa_02-vnf_with_vim_metrics_and_autoscaling.robot

@@ -127,7 +127,7 @@ Get VNF VIM-based Metric Before Auto-scaling
 Increase VIM-based Metric To Force Auto-scaling
 
     Variable Should Exist   ${privatekey}   msg=SSH private key not available
-    Execute Remote Command Check Rc Return Output   ${vnf_ip_addr}   ${username}   ${password}   ${privatekey}   for i in {1..9}; do yes > /dev/null & done
+    Execute Remote Command Check Rc Return Output   ${vnf_ip_addr}   ${username}   ${password}   ${privatekey}   for i in {1..9}; do yes &> /dev/null & done
 
 
 Wait VIM-based Metric To Exceed Threshold