Build script for 20.04 (#1004)

Should be able to build 16.04, 18.04 and 20.04,
as well as branch builds.

util/vm/install-mininet-vm.sh

@@ -15,13 +15,7 @@ sudo sed -i -e 's/splash//' /etc/default/grub
 sudo sed -i -e 's/quiet/text/' /etc/default/grub
 sudo update-grub
 # Update from official archive
-sudo apt-get update
-# 12.10 and earlier
-#sudo sed -i -e 's/us.archive.ubuntu.com/mirrors.kernel.org/' \
-#	/etc/apt/sources.list
-# 13.04 and later
-#sudo sed -i -e 's/\/archive.ubuntu.com/\/mirrors.kernel.org/' \
-#	/etc/apt/sources.list
+sudo apt-get -qq update
 # Clean up vmware easy install junk if present
 if [ -e /etc/issue.backup ]; then
     sudo mv /etc/issue.backup /etc/issue
@@ -34,11 +28,10 @@ sudo apt-get -y install git-core openssh-server
 git clone git://github.com/mininet/mininet
 # Optionally check out branch
 if [ "$1" != "" ]; then
-  pushd mininet
-  #git checkout -b $1 $1
-  # TODO branch will in detached HEAD state if it is not master
-  git checkout $1
-  popd
+    pushd mininet
+    git fetch origin $1
+    git checkout $1
+    popd
 fi
 # Install Mininet
 time mininet/util/install.sh