vm_clean: delete keys from from /etc/ssh/ before shipping vm

It's a bad idea for all Mininet VMs to share the same SSH keys.
Certainly users can regenerate their own keys, but it's better
if we don't ship a key and simply regenerate it on boot.

util/vm/install-mininet-vm.sh

@@ -43,7 +43,7 @@ fi
 # Install Mininet
 time mininet/util/install.sh
 # Finalize VM
-time mininet/util/install.sh -tc
+time mininet/util/install.sh -tcd
 # Ignoring this since NOX classic is deprecated
 #if ! grep NOX_CORE_DIR .bashrc; then
 #  echo "export NOX_CORE_DIR=~/noxcore/build/src/" >> .bashrc