Fix variable access deprecation errors

Referencing variables as `puppetversion`, `facterversion`, `processor0` and
`architecture` in an E-Ruby template like this has been deprecated. Instead,
they must be prefaced by an @ symbol.

Not prefacing the variables like this produces errors similar to:

> Warning: Variable access via 'fqdn' is deprecated. Use '@fqdn' instead.
> template[/opt/puppet/vendor/modules/motd/templates/motd.erb]:8