Remove unencrypted git protocol use (#1100)

Because Github removes support for unencrypted git protocol in March 2022, URLs in scripts are changed to use https.

See https://github.blog/2021-09-01-improving-git-protocol-security-github/ for details

util/vm/install-mininet-vm.sh

@@ -25,7 +25,7 @@ if [ -e /etc/rc.local.backup ]; then
 fi
 # Fetch Mininet
 sudo apt-get -y -qq install git-core openssh-server
-git clone git://github.com/mininet/mininet
+git clone https://github.com/mininet/mininet
 # Optionally check out branch
 if [ "$1" != "" ]; then
     pushd mininet