Fix apache::default_mods loading

PR #314 was merged without tests and consequently had bugs. This moves
the `apache::default_mods::load` defined resource into
`manifests/default_mods/load.pp` and adds the missing `mods` parameter
to the `apache::default_mods` class. Also lots of tests are added.
Merge pull request #348 from hunner/fix_314

Fix apache::default_mods loading