Do not add authenticators directly to authentication flows but use a conditional sub-flow

[pedroigor]

Today, authentication execution steps are added directly to the authentication flow.

While this works fine for what we need so far, it is not the ideal and we want to group those executions within a conditional sub-flow instead.

The main argument for having this is the possibility to add or change the sub-flow without changing the authentication flow itself so that we have more flexibility to change how organization members authenticate to a realm.