Auth: Use auth broker by default

[Jguer]

What is this feature?

• Enable auth broker by default.
• Add non documented option to disable auth broker for one last version

To disable the broker

[auth]
broker = false

[vtorosyan]

Note for future reader: auth broker is a new service used for authentication, despite the fact that there is no major feature/behavioural change, since the codepath is new, errors might happen. So if anything goes off with the auth scenarios, we can trace back to this change to debug further if there is any correlation.