Provide option to throttle primary rsync bandwidth

When Juju becomes unresponsive or suffers from API timeouts, it can cause the rsync fan out of fresh mirror data to fail and take a few hours to recover.

This is especially problematic for under-resourced/non-HA controllers. If the charm provide some options to throttle via rsync's bwlimit on the primary, we could tune the rate to be proportional to the speed of the u-r-c root-disks, saving IO bandwidth so Juju API requests don't have to compete so heavily with big rsync replications from upstream mirrors.