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 
@tenderlove / @Fryguy 
the concern FixAuth::AuthModel bring in the "hardcode" method.
Is there a way to override that method. I tried alias_method, alias_method_chain and super, but was not able to leverage the existing "recrypt" method.

ideas?