SSH configuration

Created by: exarkun

Fixes #34 (closed)

This also includes a fix for a regression introduced by #31 . I should split it out but I guess I'm not going to because it would take more work. Sorry. #31 caused systemd to create the Tahoe-LAFS storage node directory and put an empty logs directory in it. This causes Tahoe-LAFS create-node to fail because it does not want to operate on a non-empty node directory. The fix is not great. I'd like systemd to not create the directory, only delete old stuff from it if it finds some. I don't think systemd has that feature though. Someday we could implement our own cleanup service and use it instead.