MODULES-3838 Pass mod_packages through init.pp to allow overrides

[optiz0r]

When using the SCL packages on CentOS, package names have a prefix that needs to be overridden. With most other attributes in params.pp, this is possible by overriding variables passed to Class['::apache'], but in the case of mod_packages it's read directly from the params file on use. This is workaroundable by not using the apache::mod::... subclasses can calling apache::mod { 'mod_name' } directly, but there is a compounding problem.
The apache::vhost::auth_kerb parameter is used for for both including the auth_kerb modules (using class syntax rather than include, so cannot be duplicated), and for including config snippets in the generated vhost. It's not possible to make use of the config snippet whilst also needing to amend the package name.

This commit follows the existing style and passes mod_packages through init.pp, and updates all references of apache::params::mod_packages to apache::mod_packages. It's therefore possible to override the package names using something like:

include ::apache::params
class { '::apache':
  mod_packages => merge($::apache::mod_params {
    'auth_kerb' => 'httpd24-mod_auth_kerb',
  }
}

[eputnam]

@optiz0r thanks for the PR! could you do a quick rebase? also, it'd be good to have some tests and documentation for this as it's a new feature.

[optiz0r]

Rebase done. I don't have a lot of free cycles, and considering there is no documentation or tests for the apache::params::mod_packages in the first place, there's nothing to update, and the codebase is no worse off by any measure with this commit as-is than it was before.