Fix list_consumer_groups() to query all brokers

Previously, this only queried the controller. In actuality, the Kafka
protocol requires that the client query all brokers in order to get the
full list of consumer groups.

Note: The Java code (as best I can tell) doesn't allow limiting this to
specific brokers. And on the surface, this makes sense... you typically
don't care about specific brokers.

However, the inverse is true... consumer groups care about knowing their
group coordinator so they don't have to repeatedly query to find it.

In fact, a Kafka broker will only return the groups that it's a
coordinator for. While this is an implementation detail that is not
guaranteed by the upstream broker code, and technically should not be
relied upon, I think it very unlikely to change.

So monitoring scripts that fetch the offsets or describe the consumers
groups of all groups in the cluster can simply issue one call per broker
to identify all the coordinators, rather than having to issue one call
per consumer group. For an ad-hoc script this doesn't matter, but for
a monitoring script that runs every couple of minutes, this can be a big
deal. I know in the situations where I will use this, this matters more
to me than the risk of the interface unexpectedly breaking.

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