New CentralService REST APIs for DataService lookup

[dohyunkim-dev]

I propose to add one more REST API for CentralService-DataService interoperation. Currently, we cannot query the list of DataServices registered to a specified CentralService. Current implementation assumes that a user knows the list of DataServices to add a building/sensor to the DataService. This means that a user should hold the global state of the DataServices connected to the CentralService or access the Web UI for the list of DataServices. Adding a new, simple API that returns the list of DataService names can enhance the usability of REST API. Or, we can also add search API for data services (ex, DataService search using hostname, etc)

[jbkoh]

An upvote for just listing dataservice. I think a user can parse them out for what she wants.