New definition for managing systemd unit files.

This adds a new definition, systemd::unit, for managing unit files
in /etc/systemd/system.  The content of the unit file can be given
as a hash of hashes (like in the systemd::unit_options definition),
as a string, or as a path or puppet: URL to a file to copy.

Since we use the same ERB template file for formatting the hash of
hashes into an ini-style file in both systemd::unit_options and
systemd::unit, we have renamed the template file to "unitfile.erb",
to be more neutral.  The comment at the end mentioning from where
the file is managed, now takes the Puppet definition as a parameter,
set by the definition.