Refactor params

Refactors the module so that the `params` class is only used to set variables where the values may differ between operating systems. All parameters with static values are set with default values on the top `nginx` class, from which `nginx::config` and `nginx::package` now inherit.

Since they now inherit from `nginx`, there's no need to explicitly declare the classes in `init.pp` so simple `includes` are used instead.

This simplifies the module by removing about 150 lines of redundant parameter setting and will allow for easier modification in future. All tests pass without making any changes to the test suite, except for making a couple of extra facts available to a few tests due to the new dependencies.