WIP on a doc for adding services
Lots more documentation on adding services
how to enable the new service in the CLI
covering specs for cli helpers before refactoring
Move ServiceConfig building code into SystemConfig
Refactor postgresql/redis service classes into common superclass
SystemConfig is setup via the ServiceConfig subclasses
Move service_classes to top of method for easy discovery
Create SystemConfig accessor methods for services based on ServiceConfig classes
Additional docs on what SystemConfig accessors are created
Can restrict supported services via SystemConfig#available_services array
Simpler interface btw renderer and config objects - #build_into_manifest method
render no longer has to know what services are supported

SystemDeploymentManifestRenderer delegates to SystemConfig for the list
of supported ServiceConfig objects