Increase the alerting ceiling for rsync processes

Hi

The charm sets "max connections" for rsync, and then deploys a nagios check to ensure that we don't go over that figure.

However, it seems like we'll often see slightly more processes than the threshold, presumably due to the order in which rsync forks child processes and checks its own constraints (guessing it doesn't check first!).

Currently if we set up for e.g. 64 max connections, the check will WARN at 65, and CRIT at 70.

Let's consider bumping those a bit, maybe bumping WARN to 72 or so, and CRIT to 80.

It may also make sense to tackle LP:1891103 at the same time here.