Add Kafka rack awareness/Fetch from follower to Kafka source configuration

[dpavlov-smartling]

This feature will make Kafka cross availability zone traffic in any cloud hosting provider free of charge. This is a huge bill than you process a lot of logs and your Kafka infrastructure is spread between multiple availability zones.
LibrdKafka library does support this as per https://docs.confluent.io/platform/current/clients/librdkafka/html/md_INTRODUCTION.html#fetch-from-follower , but we can't use this configuration, because QW assigns Kafka pipelines on its own and we can't just hardcoded Kafka client rack parameter in Kafka Source configuration like this:

version: 0.8
source_id: my-kafka-source
source_type: kafka
num_pipelines: 2
params:
  topic: my-topic
  client_params:
    bootstrap.servers: localhost:9092
    client.rack:us-east-1a

Kafka source configuration should support fetching client.rack from the environment variable on the QW indexer server.
For example you can use the following configuration of the source:

version: 0.8
source_id: my-kafka-source
source_type: kafka
num_pipelines: 2
params:
  topic: my-topic
  client_params:
    bootstrap.servers: localhost:9092
    client.rack:${QW_KAFKA_CLIENT_RACK}

Now when pipeline is scheduled on QW Indexer with env variable QW_KAFKA_CLIENT_RACK set to "us-east-1a" Kafka source config will take it from that variable and set as a client.rack parameter during connection to the Kafka server.