Refactored protobuf framework, added loader

Included changes:

* Initial adapter interface spec
* Adapter loader to auto-load adapters
* Initial adapter NBI
* Better import support in chameleon, allowing more flexible
  protobuf structure (by organizing artifacts and service
  into separate proto files.
* Refactored voltha.proto to split things into logical proto
  modules.
* Some additional cleanup

Change-Id: I75f9883c6992148ea8df430bcdaebf85115fea4a
4 files changed