Add Auth/Encryption models

[Fryguy]

Password Mixin should probably just go to manageiq-password - See ManageIQ/manageiq-password#6

[Fryguy]

While this was technically already implemented directly, not every app that mixes in this gem will want this feature. We need to find a way to make models and their migration set opt-in. On option I'm thinking of is to just store the models in subdirectories, then the calling app would need to add those model and migration directories to their app paths. So for example,

• app/opt_in_models/auth/authentication.rb
• app/opt_in_models/auth/encryptions.rb
• db/migrate/opt_in_migrations/auth/20180123123456_add_authentications.rb

Then in the app

config.autoload_paths << ManageIQ::API::Common::Engine.root.join("app/opt_in_models/auth").to_s

and for the migrations something like: https://github.com/ManageIQ/topological_inventory-core/blob/270173af2490089f3f7b30862497dca5d150c7cf/lib/topological_inventory/core/engine.rb#L7-L13

@bdunne @syncrou Thoughts? Maybe we do this for some features like Auth which are optional, but other features like Tenancy we can enforce by just putting stuff in app/models and db/migrate.

[syncrou]

I like this idea @Fryguy, do you think we would need to set up a procedure for documenting what exactly is opt_in ( other than looking at the opt_in_models directory? ) Most likely just requiring details in the README would be good enough.

[Fryguy]

Not sure...maybe there can be helper methods, but I haven't thought it through.