Apache service

This PR adds $apache_service parameter to apache::params class and $service_name parameter to apache::service class. These parameters are used by #342 and #343. This PR should be merged after #435. The purpose of the whole thing is to split #342 into parts to make review process easier, see https://github.com/puppetlabs/puppetlabs-apache/pull/342#issuecomment-25423813.