Enable different configuration for different environments

As a developer, I want to be able to provide different configuration to different environments, so that I can handle the different needs of the different environments such as dev, integration and production.

Context

Configurations can be supplied in light modules or Java modules. Typically, the same definition configurations are used in all environments, with the only thing changing being the new configuration that is being tested. 

However in some cases we want different configurations in one or more of these environments, for example we may want a restClient to point to a sandbox API while in development, but point to a production API while in production. Or on a site definition, we might want to use differnet domain names based on environment.

Bigger Picture

Consider the related need of applying differnet configurations to author and public instances within each of those environments as well.

See: https://jira.magnolia-cms.com/browse/MAGNOLIA-7882