Allow for a default load order as a convention

[rfdickerson]

Configuration allows for a very rich way to specify multiple sources for configuration information with priority ordering. However, for many applications, a convention that does the equivalent of:

let config = ConfigurationManager
                                 .load(file: "config.json")
                                 .load(.environmentFiles)
                                 .load(.commandLineArguments)

So that the user simply has to do:

let config = ConfigurationManager.default()

[rfdickerson]

@rolivieri came up with this nice way of loading the cloudConfig file locally and properly on Bluemix:

configMgr.load(file: "../../\(cloudConfigFile)")
configMgr.load(file: cloudConfigFile, relativeFrom: .pwd)

[rolivieri]

All, just sharing some additional context:

let cloudConfigFile = "cloud_config.json"
…

configMgr.load(file: "../../\(cloudConfigFile)")
configMgr.load(file: cloudConfigFile, relativeFrom: .pwd)

where cloud_config.json is found in the root folder of the repo. For instance, see: https://github.com/IBM-Swift/swift-enterprise-demo. If we can come up with a way we can abstract this logic inside CloudConfiguration, that would remove the need for this boilerplate code in the app.