Control CLI via env variables

I propose following CLI modification (considering only version 3.x)

  • support NAMEKO_CONFIG env variable as alternative for --config option. Value would specify (single) path to a config file such as "config.yaml". Applicable to nameko run, nameko shell and nameko shell-config.
  • support NAMEKO_SERVICES env variable as alternative for declaration of modules/service classes to run. Value would be space delimited list of module[:service class] strings such as "myapp.service:publisher myapp.monitor". Applicable to nameko run.

With such env var. tooling, it would be easy to set env variables and start an app simply by nameko run.

If we agree on this, I can take care of implementation (for nameko 3.x only).