Pass the --force flag to the Lizzy Agent

[boure]

At the moment, the force flag, while specified in the CLI API, is not passed to the Lizzy Agent and its usage will not work, for instance if the instance already has traffic resulting in the following error message:

ERROR: Error executing command.
> command: senza delete --region eu-central-1 my_application ABCDEFG
> command.output: Error: Stack my_application-ABCDEFG has traffic!                    
Use --force if you really want to delete it

The corresponding TODO is here:

lizzy-client/lizzy_client/cli.py

Line 445 in eaa9860

# TODO pass force option to agent

The corresponding API in Lizzy is here:
https://github.com/zalando-stups/lizzy/blob/4c39fce2617cc60be512a1f0587727b041d67e52/lizzy/api.py#L258