Set up rsync between tb-build-01 and tbb-nightlies-master

I think to set up rsync between tb-build-01 and tbb-nightlies-master we can:

• generate an ssh key on tb-build-01 with the tb-builder user
• on tbb-nightlies-master, update /etc/ssh/userkeys/tbb-nightlies with this ssh key, and a command="/path/to/rrsync ..." option to restrict access to one directory