This permits users to specify a single client property without having to
also specify the brokers the client can connect to.  This is helpful
when using docker and passing something like `--set
pandaproxy_client.produce_compression_type=lz4` to `rpk redpanda start`.

NOTE: Kafka endpoint discovery will NOT occur if you specify a client
config.  Once you specify one entry, you must specify the brokers entry
if the Kafka API is not `127.0.0.1:9092`.

Signed-off-by: Michael Boquard <michael@redpanda.com>
(cherry picked from commit d3fe0f2)