nixos/rabbitmq: fix restarts and sasl logs

1. The chmod 400 with the preset cookie prevented restarts, as
on the second boot it would fail to write to the cookie. Oops.

2. As far as I can tell, sasl logs were disabled because of the
following error:

{error,{cannot_log_to_tty,sasl_report_tty_h,not_installed}}

Not because we actually wanted to disable them. This meant the
management plugin wasn't usable due to a bug set to be fixed in
3.7.0.