add missing system roles

for cassandra test hosts, sodium (mailing lists),
    tridge (backup)

since Ie8a53c7f478401b53444e8f187243f9c41603054 we now have
automatic salt grains set by the system role

when checking the results of that (thanks Ariel), it is now
obvious how some of our nodes don't have any system roles yet

this is adding a few of them, but there will be more,
specifically labs

Change-Id: I423d798c0f4df7d06bc685316e796b67c0941dae