Allow Apache service not to be managed by Puppet

Currently Apache can either configured to be 'running' or 'stopped'. In our cluster setup Apache is managed by Pacemaker, so Puppet should not interfere. Setting ensure to 'UNSET' will now set ensure to undef for the Apache service.
The apache class now accepts true, false, 'running' or 'stopped' for the parameter service_ensure. Any other value wil result in the parameter 'ensure' for the Apache service being set to undef.
Whoops
Merge pull request #690 from arnoudj/master

Allow Apache service not to be managed by Puppet