Allow other manifests to define ::apache::mod{ 'ssl': }.

This will allow Apache from the httpd24 SCL on RHELish systems to
include the proper mod_ssl package while still letting ssl
::apache::vhost defines work as expected.

Add package_name param to ::apache::mod::ssl.

Add a test for the package_name parameter.

Update the package_name parameter test.
Merge pull request #818 from smerrill/feature/allow-mod-ssl-customization

Allow other manifests to define ::apache::mod{ 'ssl': }.