Move VMs management back to vagrant

The Vagrantfile now allows vagrant to work with a variable number of VMs
ES configuration files are now dynamically generated from a template

Configuration files are generated once so the user can then customize them

IF the user wants to use his own configuration files, he has to put them in conf/<vm_name>/elasticsearch.yml before starting the VM
Merging and updating small things but problems with screens, root user and /usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
Tweak sudo configuration to preserve some env. variables
Store ES logs in the vagrant synced folder to ease reading it.
Allow the user to override the cluster IP range with CLUSTER_IP_PATTERN env. variable

For example, running: `CLUSTER_IP_PATTERN=192.168.50.%d vagrant status`
will configure the cluster with the `192.168.50.0` network