Simplify support for other storage options

[sbuettner]

The two new resources under the /storage/v1/diag path which are provided by the StatusStorageResource and TreeStorageResource implementations make strong assumptions about the storage system in use since they only use the folderPath of the configured filesystem. Before the introduction of these resources it was quite easy to add support for other storage options (S3 for example).

Would it be possibly to provide an abstraction that might be used by the resources under /storage/v1/diag to allow other storage options? I was thinking about something like a DiagnosticService interface which could be implemented for other storage options.

Cheers

[chook]

Before we introduce an interface refactor it is possible to just NOP the two diagnostics resources StatusStorageResource.java can return only the collectMachineInfo part and TreeStorageResource.java can just return an empty string "".