Replace Viper config with the local structure

[tzununbekov]

Viper config replaced with local pkg/config module. This approach provides more flexibility in control over configuration structure, upgrade process, and readability.
Additionally, Redis broker parameters support was added and the default broker version was switched from the static latest to the current latest (according to github releases page) to avoid unwanted upgrades.

Broker's image parameter is replaced with the version for consistency; configuration attributes will identify which broker to use, eg:

context: bar
triggermesh:
    version: v1.23.0
    broker:
        version: v1.0.0
        redis:
            address: 127.0.0.1:6666
            username: foo
            password: bar
            skip-verify: true

-redis broker v1.0.0 will be used

context: foo
triggermesh:
    version: v1.23.0
    broker:
        version: v1.1.0
        memory:
            buffer-size: "100"
            produce-timeout: 1s

-default configuration with in-memory broker

Related to #87.