Allow setting kcat configuration directly from environment variable

[SoerenHenning]

According to the Readme, it is currently possible to specific an environment variable KCAT_CONFIG, which specifies the path to a file in which a configuration can be found.

It would be nice, if we were able to set the configuration directly as environment variable. Especially CI pipelines or Docker Compose testing setups could be simplified, we would not need to write the config to a file first. An example for Docker Compose could look like this:

version: '2.2'
services:
  kcat:
    image: edenhill/kcat:1.7.1
    environment:
      KCAT_CONFIG: | # This is not possible at the moment
        metadata.broker.list=kafka:9092

Setting configuration options this way is, for example, possible with Apache Flink.