Exposed $logroot_mode in init.pp

This commit now allows you to set the directory permissions on the $logroot
directory. It also resolves a catalog compilation issue when specifying the
apache class with default_vhost => false, and no other vhost resources.
I've submitted a latest PR with $logroot_ensure, but I'm not sure on what it buys us exactly. Wouldn't the behaviour be the same without it? It will still create the log directory even if you're marking the vhost as "absent", and even if you set `logroot_ensure => false` it won't delete the directory. As @igalic suggested, should the value for `ensure =>` on the logroot resource now be  a selector with values based on what $logroot_ensure is set to (true => 'directory', false => 'absent')?