Use iftop to collect network bandwidth usage (#6560)

* Use iftop to collect network bandwidth usage

* fix shellcheck

* more shellchecks

* review comments

net/scripts/install-iftop.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+#
+# Rsync setup
+#
+set -ex
+
+[[ $(uname) = Linux ]] || exit 1
+[[ $USER = root ]] || exit 1
+
+apt-get --assume-yes install iftop