Use the binder minPartitionCount property for consumers

Fixes #495

Currently, `minPartitionCount` is available both as a binder default and as a consumer property.
It would be simpler if it only was a binder setting (seeing as it has effect only as a default).

spring-cloud-stream-docs/src/main/asciidoc/spring-cloud-stream-overview.adoc

@@ -839,6 +839,10 @@ Mutually exclusive with `offsetUpdateTimeWindow`.
 Default: `0`. 
 spring.cloud.stream.kafka.binder.requiredAcks::
   The number of required acks on the broker.
+spring.cloud.stream.kafka.binder.minPartitionCount::
+  The minimum number of partitions expected by the consumer if it creates the consumed topic automatically.
++
+Default: `1`.
 
 ==== Kafka Consumer Properties
 
@@ -859,10 +863,6 @@ startOffset::
 Allowed values: `earliest`, `latest`.
 +
 Default: null (equivalent to `earliest`).
-minPartitionCount::
-  The minimum number of partitions expected by the consumer if it creates the consumed topic automatically.
-+
-Default: `1`.
 enableDlq::
   When set to true, it will send enable DLQ behavior for the consumer.
   Messages that result in errors will be forwarded to a topic named `error.<destination>.<group>`.