change nfs::client::mount's parameter $mount to default to the declared instance's $title

This makes nfs::client::mount defined type more "user friendly" when declaring
NFS mounts explicity instead of gathering exports with a collection.  Eg.

    nfs::client::mount { '/d1/ftp/Volumes/noaocache':
      ensure  => mounted,
      server  => 'foo.example.org',
      share   => '/export/bar',
    }