Remove unnecessary scope.lookupvar() for nx_logdir since it's defined in
nginx::config.  The function is still necessary in
templates/vhost/vhost_header.erb though.
Follow module convention where variables have a leading 'nx_' in
nginx::params but not in nginix or nginx::config.  Commit effectively
does s/nx_logdir/logdir/ in those two classes.  Also chased the variable
change in the ERB templates.