Use environment variables for producer, consumer, discovery and metrics ports

[jorgebay]

Currently, ports for producer, consumer, discovery and metrics interfaces are defined as constants.

We should get the port value from environment variables, similar to the gossip port:

• https://github.com/barcostreams/barco/blob/v0.5.1/internal/conf/config.go#L27
• https://github.com/barcostreams/barco/blob/v0.5.1/internal/conf/config.go#L219-L221

[jorgebay]

Done in #130 by @ArtAhmetaj !