Hide output when configuring apt retry

Change-Id: I063995d52b66e4023e360ba423684753e50a291c

Author: Chmouel Boudjnah

stack.sh

@@ -213,7 +213,7 @@ sudo mv $TEMPFILE /etc/sudoers.d/50_stack_sh
 
 # For debian/ubuntu make apt attempt to retry network ops on it's own
 if is_ubuntu; then
-    echo 'APT::Acquire::Retries "20";' | sudo tee /etc/apt/apt.conf.d/80retry
+    echo 'APT::Acquire::Retries "20";' | sudo tee /etc/apt/apt.conf.d/80retry  >/dev/null
 fi
 
 # Some distros need to add repos beyond the defaults provided by the vendor