node: better fix for gh-304

The previous fix simply blasted all /etc/exports entries for each node
before exporting paths to nodes. This can potentially kill other export
paths not being exported by export_fs_to_nodes given that they will not
be redefined. Added `paths` kwarg to Node.stop_exporting_fs_to_nodes
that causes only the specified export paths for each node to be removed
from /etc/exports instead of all entries for each node.

Thanks to @realoptimal for the catch:

https://github.com/jtriley/StarCluster/commit/f7c4967ed891c245cc6d8c64ac38f9fdb345ffd6#commitcomment-4640990