Test immutable state

[SpComb]

Many of the state objects use Object#freeze to enforce immutability for shared (Observable) state, as well as enforcing idempotency for the Policy context.

• Kontena::Registrator::Configuration::State@configs#freeze
• Kontena::Registrator::Configuration::State#freeze
• Kontena::Registrator::Policy::LoadContext#freeze (after loading)
• Kontena::Registrator::Policy::ApplyContext@config#freeze
• Kontena::Registrator::Docker::Container#freeze
• Kontena::Registrator::Docker::State@containers#freeze
• Kontena::Registrator::Docker::State#freeze
• Kontena::Etcd::Model::Collection@objects[...]#freeze

These should have some kind of specs to make sure that they actually work as intended.

[SpComb]

The Kontena::Registrator::Policy cases now have specs.