Support updating Jazz core configuration via an API

[bleggett]

Description

Currently all/most Jazz configuration is driven by a JSON file that is checked into whatever configured Git repo we have at install time.

If anyone or anything wants to programmatically update this config file after install time, knowledge of the specific Git URL, file path, git credentials, config file format, and other things are required.

Our forays into feature installers in jazz-installer are starting to point this area out as a weak spot, every feature installer needs to collect that information.

It seems as if this is pointing us to a logical extension point: Some sort of core function API that allows you to programmatically update Jazz config dynamically, either by updating that config file in the repo behind the scenes, or by some other method.

[suryajak]

Good place to start with is to extend the Admin API and write code for PUT.