Make voicemail.conf into a template

It is now possible to pass a hash to the main class to override default
values or to define new ones.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
default value for $package_name is not appropriate

why was that even set to an array?

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
README: Document how to pass in general options for voicemail

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
Fix arrow alignment in README and in params.pp

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>