Currently one has to remove and re-add devices when the region or zone needs to
be changed. Adding the subcommands set_region and set_zone simplifies this, now
it is possible to change the region and/or zone easily. Note that there is no
change to the required rebalancing; it is likely that data still needs to be
moved within the cluster.
This is mostly copy-n-paste of the existing set_weight subcommand and adopting
tests accordingly. Some duplicated code in the tests has been aggregated as
well.
Change-Id: I37dd8e8ac24e2b0bb196758449a1d91a769b2e96
98f9781096