If we don't generate tables even when they're going to be empty, then we have to
figure out how to remove the 'transport_maps' and 'smtp_sasl_password_maps'
config lines that point at them. Postfix is fine with empty tables, so this is
the simplest way to solve the problem of undefined lookup mappings.

The alternative is to set the 'transport_maps' and 'smtp_sasl_password_maps'
properties to nothing, which Postfix is also fine with, or remove them entirely.

Looping over a list with the 'apt' and 'dnf' modules is deprecated.

* refactor-to-improve-flexibility:
  Update the README
  Add support for Fedora, Redhat, and Centos
  Refactor the task that installs OS specific packages
  Refactor the lookup tables to be more flexible
  Refactor the lookup tables to be more flexible and DRY
  Change the naming scheme for OS specific files

Now the task installs the OS specific required packages, plus a list of
additional packages that are defined in the 'postfix__recommended_packages'
variable.

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.

This takes the templates for the transport map table and sasl_passwd lookup
table and replaces them with a single template. The templates were already
nearly identical and we would have needed to create more duplicates for every
new lookup table we implemented.

Also, the 'postfix__transport_map' and 'postfix__smtp_sasl_password_map'
variables have been replaced with a single dictionary 'postfix__tables' where
each key in the dictionary is the name of the lookup table, and the value is a
dictionary of entries for that lookup table. By using a dictionary we can
implement default lookup tables per operationg system and then allow downstream
additions or overrides via combining the dictionary with a new dictionary
instead of having to redefine the entire thing to override a single value.

* sasl-support:
  Only place the transport_map file if we're using transport maps
  Removing the generic 'make postfix' handler.
  Adding the transport_maps attribute to main.cf so that it actually works
  Adding support for SASL authentication

There's no reason to make all the map files at once, and doing so removes any
granular feedback.