Support ZooKeeper groups

[deric]

The following example shows how to configure a system with three groups of three servers each:

    group.1=1:2:3
    group.2=4:5:6
    group.3=7:8:9

    weight.1=1
    weight.2=1
    weight.3=1
    weight.4=1
    weight.5=1
    weight.6=1
    weight.7=1
    weight.8=1
    weight.9=1

When running the system, we are able to form a quorum once we have a majority of votes from a majority of non-zero-weight groups. Groups that have zero weight are discarded and not considered when forming quorums. Looking at the example, we are able to form a quorum once we have votes from at least two servers from each of two different groups.

from ZooKeeper docs.