Ensure cobra command isolation

[mflendrich]

There's a new class of quality issues in deck (see #331 and #288): cobra Commands wrongly overwriting or reading values meant for other commands.

I propose that we improve maintainability/modularity of decK by refactoring cobra commands so that:

• each lives in a separate package
• each defines a struct Config and has flags bound to a single instance of that struct
• implementation of a command is decoupled from cobra, and takes the aforementioned struct Config as its input

I did something similar on one of my past projects: https://github.com/weaveworks/wksctl/blob/0184323ebc93df96544a1e3b55c84fbd3d292641/cmd/wksctl/main.go#L47

[mflendrich]

Notes after a discussion with @hbagdi:

• We need to ensure no regression for users configuring decK with either a config file or env variables (as supported by viper)
• Currently viper binds flags to env vars on the global level - this may need some rearrangement.
• Env variables (that are not settings) are also used in feat(file) implement env var substitution for state files #286 and whatever we implement, that needs to work with env var substitution implemented in that PR.