Refactor runtime property configuration

Detailed Description

We'd like to add support for specifying runtime configuration properties via CloudFoundry user-provided services.

For example, an operator would specify prior to deployment:

cf create-user-provided-service stratos-properties -p '{"CF_CLIENT": "stratos","CF_CLIENT_SECRET": "xxxx"}'

And then in the manifest.yml:

applications:
  - name: console
    ...
    services:
    - stratos-properties

We've found this pattern works better than other obvious alternatives with CloudFoundry because:

1. Putting secrets (such as the client secret) in the manifest.yml as an environment variable is challenging with respect to public source repositories.
2. Setting environment variables on an already running app doesn't work well as many zero-downtime deploy plugins create a new app on each deployment.

Context

Currently runtime properties are spread between:

1. ./config.properties file
2. Environment variables (sometimes set via manifest.yml)
3. /etc/secrets directory

Some settings appear to only work if specified in the ./config.properties file, others appear to allow overriding by environment variables.

Possible Implementation

Last year we made a small Go library to handle looking up environment variables from a list of sources, e.g. first look for environment variables, falling back to data provided by a user provided services, falling back to defaults.

We (cloud.gov.au team) worked with 18F to integrate this with their console.

I've made an experimental change to integrate the same library with the Stratos codebase which is linked as a PR below.

The change is actually quite small - the main part (in main.go) is this method, which initialises an env.VarSet:

// getEnvironmentLookup return a search path for configuration settings
func getEnvironmentLookup() *env.VarSet {
	// Make environment lookup
	envLookup := env.NewVarSet()

	// Environment variables directly set trump all others
	envLookup.AppendSource(os.LookupEnv)

	// If running in CloudFoundry, fallback to a user provided service (if set)
	cfApp, err := cfenv.Current()
	if err == nil {
		envLookup.AppendSource(env.NewLookupFromUPS(cfApp, "stratos-properties"))
	}

	// Fallback to a "config.properties" files in our directory
	envLookup.AppendSource(config.NewConfigFileLookup("./config.properties"))

	// Fallback to individual files in the "/etc/secrets" directory
	envLookup.AppendSource(config.NewSecretsDirLookup("/etc/secrets"))

	return envLookup
}

which is passed around to each component that needs to lookup anything from any of the config sources (that is, it is a replacement for the the config.GetValue() calls).

The branch above compiles and pushes, I haven't tested any deeper than that yet, but we can look at if this is a direction worth pursuing.

Start with main.go and the components that currently use config.GetValue(), then read the env.VarSet integration and the existing config.properties and /etc/secrets lookup paths. Done means the runtime configuration sources have a defined precedence, including Cloud Foundry user-provided services, and the affected components use the shared lookup consistently.