[Reviewer: Andy] Don't destroy /etc/clearwater/config if sed expression is bad

Andy, please can you review my fix?  Someone turned up an all-in-one node on a non-DHCP network.  We don't expect this to work.  Unfortunately, this caused a bad sed expression, which meant that we wrote nothing to /tmp/clearwater.config.$$.  We then moved this back over /etc/clearwater/config, corrupting it.  Instead, I now use -i to edit /etc/clearwater/config in place.  In particular, this means that if the sed expression is bad, we don't touch /etc/clearwater/config.  I've tested this live.