When a consumer tries to locate a consumer group coordinator of a cluster
for the first time, the __consumer_offsets topic is created with the
number of partitions as per the group_topic_partitions property.
The default value for that property was 1 which means that unless
a different value was explicitly specified by the customer at a very
early stage of cluster's life, all OffsetCommit requests from all
consumers will be going to a single broker. This change increases
the default value to 16 as a reasonable trade-off between OffsetCommit
parallelism for the clusters that will use consumer groups
later in their life, and the overhead for the clusters that
won't use consumer groups.

redpanda-data#5222

test_leadership_transfer relies on kafka_group_offset metric source as an
indication of the node being a group coordinator. However in admin API
there is no way to know which __consumer_groups partition a specific
consumer group uses, and the testcase always works with partition 0.
To make that work, number of __consumer_groups partition is set to 1
explicitly.