Clean up foreman::config

This removed all requires on install and notifies to service since init
should take care of that relation.

To remove OS specific information, it moves init_config and
init_config_tmpl to params.pp.

Since the crons are no longer defined, setting the default is no longer
needed.
adding support for plugins adding virtual host entries
Refs #3308 - Ensure plugins are installed after core
Refs #3308 - Add plugins repo to repo management