Allow environments to inherit and override config from other environments

[nickthecook]

Sometimes it's useful to have one environment inherit the config from another environment and just override a few variables.

Ops could provide a feature that uses declarations like this in ops.yml:

environments:
  dev:
    depends_on: [test]

In this case, Ops would load the test environment config (and secrets, if applicable), then load the dev environment's config over that, allowing the second environment to override variables defined in the first environment.

[nickthecook]

Adds to much complexity, and the duplication is not a problem in practice, even for terraform+ansible+app projects.