Nailgun objects should use FSM to simplify business logic

Right now every object we're working with at any moment of time has it's exact state, which is clearly defined by some of it's fields.

For example, each Node is exactly defined by three fields: state (including intermediate, like "pending_addition"), it's roles and online/offline. UI shouldn't have any logic like "if it has this, this and that flag, we may remove it from cluster this way, but if also this - then another way".

So, let's use FSMs together with rewriting our code to using REST objects.