apache: add apache::mpm

Adds a class that lets you select an Apache Multi-Processing Module (MPM).
Prefork, worker and event are available. Prefork is the default. Trusty and
Precise are both seamlessly supported.

Tested via Vagrant on both Precise and Trusty.

Change-Id: Ic952146b5eecb1d2e163e91fc08232202c728249