add $root_group parameter

[ptomulik]

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 #342 (comment).

[igalic]

You're not adding any include apache::params to those classes, is this going to work cleanly ?

[ptomulik]

Some of these manifests require apache class to be present (mod.pp, vhost.pp) which derives from apache::params. Other manifests are MPM modules, which are (always?) included from apache, so apache::params should already be present. These MPMs already used variables from apache class (apache::mod_dir) without without demanding explicitly the apache class to be included so it should not be any problem to use variables from apache::params as well.