Fix partition issue in ensemble_sync

It was previously possible for the 'minority' network partition to
become the majority network partition by a naive network partitioning
strategy. Previously, when a preference list of 5 keyspace partitions
was created on only four distinct nodes, it became possible for a 2 node
'minority' network partition group to actually have a majority of
keyspace partitions because 2 keyspace partitions were assigned to 1
node in the 'minority' group. This was fixed so that the 'majority'
group now always has a majority of keyspace partitions by preventing
nodes with greater than 1 keyspace partition from becoming part of the
'minority' group.