Simpler Makefile targets.
Stop using puppet for the desktop image creation.

Puppet doesn't seem to have any way of displaying the progress of the
ubuntu-desktop package installation, which takes a long time, so
vagrant just looks hung while that is installing. Also, it seems quite
error-prone, eg, timing out instead of completing correctly.

So in this commit I revert to simple shell commands to install
ubuntu-desktop, which vagrant beautifully prints on stdout, making it
easy to watch the package installation process.

Note that puppet is still used during `make launch-*`.