Refactor the lookup tables to be more flexible

Instead of provisioning the main.cf file from a template, we now modify the
existing file. This makes it easier to support multiple operating systems and
Postfix versions.

Also, all of the variables that were being used by the main.cf template have
been replaced with a single dictionary that defines properties and the desired
value. This means that we don't have to define all the default values, and only
have to define the specific non-default values necessary.