add $root_group parameter

On some OSes (FreeBSD, for example) the root group's name is other than 'root'. This PR introduces $root_group paramater to deal with such situation. The $root_group parameter is used by #342. This PR is created to split #342 into parts to make review process easier, see https://github.com/puppetlabs/puppetlabs-apache/pull/342#issuecomment-25423813.