Return correct number of changed partitions

When a ring is rebalanced the number of changed partitions is counted.
Before this patch partitions might be rebalanced, but actually no data
is moved - for example, when a partition is assigned to the same device
as before. This results in a wrong number of reassigned partitions that
is shown to the user.

This patch remembers the partition allocation before the rebalance, and
compares it to the new allocation after a rebalance. Only partitions
that are stored on a different device than before are counted.

Partial-Bug: 1367826
Also-By: Florent Flament <florent.flament-ext@cloudwatt.com>
Change-Id: Iacfd514df3af351791f9191cef78cff1b3e2645f
Merge "Return correct number of changed partitions"