Update admin guide on handling drive failures

Simply replacing a failed disk requires a very long time if the ring is not
changed, because all data will be replicated to a single new disk. This extends
the time to recover from missing replicas, and becomes even more important with
bigger disks.

This patch updates the doc to include a faster alternative by setting the weight
of a failed disk to 0.  In this case the partitions from the failed disk are
distributed and replicated to the remaining disks in the cluster, and because
each disk gets only a fraction of the partitions it's also much faster.

Change-Id: I16617756359771ad89ca5d4690b58a014f481d9b
Merge "Update admin guide on handling drive failures"