introduce the 'user-custom' configset directory

[JaydenLiang]

Scenario:
The project provides a set of pre-configured configset files by default for the initial deployment. However, in the real life, users should be able to adjust the initial deployment to meet their needs. To allow for the Autoscale project to maintain the pre-configured configset from version to version, we'd also like to extend the project beyond this point to provide the capability of user customization.

The idea is:
Autoscale will look for the 'user-custom' sub-directory within the configset location. If the 'user-custom' directory exists, all configset files inside it will be loaded.

This change should be implemented in the core level. Every platform-specific implement will inherit it without additional changes in the code.