Label selectors for compilation

[adrianchifor]

Problem

Currently we can only selectively compile targets using -t flag. In cases where we can't remember target names or there are just too many targets to input, we need an alternative.

Proposal

Similar to how you can filter Kubernetes objects by label on kubectl with -l flag, I'm proposing an additional kapitan compile flag --labels, -l.

Spec and usage

class-inherited-by-dev-targets.yml:

kapitan:
  labels:
    env: dev

class-inherited-by-prod-targets.yml:

kapitan:
  labels:
    env: prod

Only compile dev targets:

kapitan compile -l env=dev