Allow to use oslo.config instead of argparse to parse arguments

Currently cliff uses only argparse for parsing CLI arguments. Oslo.config also allows to parse arguments and it has one good feature - it can get the option from config file when the CLI argument wasn't specified.

In the current state of cliff, there is no way to nicely do a workflow "if argument not specified, get it from the config file". When allowing to use oslo.config, such a workflow would be possible.