make Concat[/etc/exports] a virtual resource

This is fix for error, that occurs when nfs server is created before
clients. Concat fails if fragments dir is empty:

  /var/lib/puppet/concat/bin/concatfragments.sh -o
  /var/lib/puppet/concat/_etc_exports/fragments.concat.out -d
  /var/lib/puppet/concat/_etc_exports returned 1 instead of one of [0]

Making Concat[/etc/exports] a virtual resource and realizing it in
nfs::export, we make sure that concat is not triggered if there is
nothing to concatenate.