Kafka topic namespacing capability based on env var

Currently kafka topics are hardcoded in kafka publisher go script https://github.com/codebgp/gobmp/blob/master/pkg/kafka/kafka-publisher.go . We need  to be able to prefix them based on env var so that we can differentiate among different BMP collectors (receiving data from multiple data services).

Blocks: https://github.com/codebgp/bgp-data/pull/455 