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 
ActiveSupport::Concern is built around include and extend, so no.  But concerns are not really all that special under the covers, they basically are plain modules.