Add remote_dir tag information to migration description
Prevent remote_dirs from being also registered as dir

The unmanaged-files inspector listed the remote-dirs also as
regular dirs, at least some times.
This commit does prevent it.
Add and activate NFS server to base images

We want to have integration tests for remote path filtering.
To test these a running NFS server environment is set up in our base
images.
Add integration test for unmanaged-files remote-dirs

and fix issues in the integration test caused by the new remote_dir tag.