Kafka topic creation happens asynchronously. This means that even when the
return value from createTopics(...) indicates that the topic has been created
successfuly, the topic can not be immediately subscribed to.

We retry DescribeTopics until all of the topics for the pipeline can be
described successfully. This indicates that the topic has been fully created at
least on one broker, and can now be subscribed to.

Which issue(s) this PR fixes:
Fixes gateway component for #INFRA-663 (internal): Pipeline creation goes into ERROR state

- timeout config now in gateway/constants.go
- reversal of the `pipeline.wg.Add(1)` code move
- adding error returns when reaching error states
- tidying-up of error reporting