using {{ domain }} instead of hardcoder lilik.it in resolv.conf.

Now xfs is working, `prepare_host.yaml` is modified to add `xfs` to
the list of modules loaded at each boot.

If module is added to that list, than is also loaded with a modprobe
handler.

If xfs is not working with `role/lxc_guest`, run the patched
`prepare_host.yaml` againg.

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.