[new feature] Creating a OXP agent plug-in implementation

[YufengXin]

This is for LC controller to plug in the OXP customized implementation of the following functions when first start up a new OXP site..

1. topology translation
2. breakdown provisioning
3. event handling (topology change due to link/node addition/deletion)

The LC reads environment variable to get the plug-in class/functions; which should be inherited and override the parent class/methods, abstract or interface that comes with the general LC software.

We make this one of this year's new development deliverables.

We can discuss the design/implementation first.