Simplify re-encrypt logic

Want to introduce the ability to decrypt with an arbitrary key so we can convert from one v2_key to another.


- separate auth_model_spec and auth_config_model_spec
- consolidate fix_auth#hardcode logic with rencrypt logic
- encrypt_version_{1,2}(str) -> encrypt_version(str, #, key)
- decrypt_version_{0,1,2}(str) -> key.decrypt64(str)
  (it was a partial implementation of decrypt)

https://bugzilla.redhat.com/show_bug.cgi?id=1159009

/cc @jrafanie @Fryguy 
One easy/obvious solution is to directly include `FixAuth::AuthModel::ClassMethods` into `FixAuth::AuthConfigModel::ClassMethods`. Inelegant, but an improvement over a copied method?

---

``` ruby
included do
  def self.recrypt(old_value, options = {})
    ..
    super(v, options)
    ..
  end
end
```

Maybe?

---

I've gone back and forth several times, but I keep convincing myself that `super` should Just Work, with the method defined in ClassMethods, with no special effort. What goes wrong when you try that? NoMethodError?