Remove the `os` setting from vcluster.conf and keep only one
image URL (which will also be removed in following patches
and replaced by either a debootstrap process or an official
debian base image from snf-image repo).
Use truncate instead of dd to create the local file
used for extra disk in the cluster's VMs.
Signed-off-by: Dimitris Aragiorgis <email@example.com>