Add a list of users to create accounts for. Installs ssh public
keys from `$user.sshkey` files in a local `keys` directory.

These files are not currently checked into the repository,
since we're unsure about publishing them.

The Debian installer defaulted to `lvm` and we're unlikely to
change it, but using a variable makes it less likely we'll
make errors if we do change it in the future.

Datacenter staff installed Debain 10.4 and left us with
ssh access to the root account, and a minimal lvm filesystem
setup.

To help document and maintain the configuration, declare
the basic storage volumes.

Assuming ssh access to root@mf4.xiph.org, the config
can be applied with:

  ansible-playbook -i hosts mf4.yaml

In theory it should verify the config, but avoid doing anything
destructive if it finds the system in a state that doesn't match
what's described.

This hasn't actually been tested on a bare system; resizing
the root and swap might need manual intervention.