Don't fall back to old command syntax in the new client.

Bugs closed: AURORA-782

Reviewed at https://reviews.apache.org/r/28771/
Use abstract decorators in CommandLine class.
This ensures the name property and register_nouns method are implemented.

Reviewed at https://reviews.apache.org/r/28696/
Simplify logging in the client.

Bugs closed: AURORA-919

Reviewed at https://reviews.apache.org/r/28742/