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.

With the transition to the new ninux-compatible network we choose to
have a ip pool for vm that resides on the 10.150.42 network.
As 254 unique addresses are sufficient for our need we reduce the subnet
to a /24