Provide proto backwards-compatibility

[remingtonc]

Protocol buffers provides backwards/forwards-compatibility via consistent field indexing/naming. Ideally the protos could be found in a normalized/ folder which provides a consistent usage experience no matter which revision we use. I recognize this might not be possible with the impedance mismatch from YANG -> protobuf and MDT infrastructure, so if this is not feasible feel free to close. Related to https://github.com/cisco-ie/cisco-proto/issues/3