Fix dashboard_{user,group} Variables

The dashboard_user and dashboard_group variables correspond to
both the system puppet-dashboard user and group as well as the
MySQL puppet-dashboard user. This commit fixes where I was using
the non-validated variable as well as adds documentation.

tests/init.pp

@@ -3,6 +3,7 @@ node default {
   class {'dashboard':
     dashboard_ensure          => 'present',
     dashboard_user            => 'puppet-dashboard',
+    dashboard_group           => 'puppet-dashboard',
     dashboard_password        => 'changeme',
     dashboard_db              => 'dashboard_production',
     dashboard_charset         => 'utf8'