Kafka consumer context bean names

 - Avoid using hardcoded bean names for consumer context
 - Remove Integer value topic streams count, instead use string value
   - This will allow placeholder values to be used for `streams` attribute
 - Set `group-id` as a bean name for consumer configuration bean only if
   the group id is specified explicitly. This will allow placeholder values
   not being used as a bean name.
 - Add test cases to verify multiple consumer configurations within the
   same consumer context.
@artembilan Using `HashMap` here works as I think it doesn't use any bean references. Do you think using ManagedMap here still makes sense?