And minor improvements/refactoring

Debian version upgraded to buster.

New templates compatibile with stable versions of LXC provided with
debian.

Cleaner syntax using `ssh_lxc.py` connection plugin. Now we don't user
`lxc-attach -n ...` in the `shell` module on the host anymore, but we
delegate to `{{ vm_name }}` with `connection: ssh_lxc`, using suitable
Ansible module to do operation directly on the container before it is
online and SSH accessible.

We added an option to force an LVM VG name: if the default naming
convention is not used the vg name can be overriden with the `vg_name`
variable.

The `xfs` filesystem seems to be broken in this release, so we used
`ext4` as default for new container. This point needs further
investigation.