(GH323) Better parameterization of root user/group from #279

[rnelson0]

Fixes #323

The parameters $root_user and $root_group were added in #279. To do so, the main class r10k was required, rather than using the values from r10k::params, which created a conflict when using some subclasses without the main r10k class; specifically when replacing it with pe_r10k. This adjustment allows fully independent usage of r10k::mcollective and r10k::webhook again. It may require adjustment of the class parameters/hiera values, however, since the value in both classes is pulled from r10k::params. Heads up @buzzdeee and other users who may have overridden that value.