This just updates the config and saves it to `flamenco-manager.yaml`.
Saving the configuration doesn't restart the Manager yet, that's for
another commit.
This adds a `-wizard` CLI option to the Manager, which opens a webbrowser
and shows the First-Time Wizard to aid in configuration of Flamenco.
This is work in progress. The wizard is just one page, and doesn't save
anything yet to the configuration.