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. First step is to simplify the recrypt chain.

- separate auth_model_spec and auth_config_model_spec
- consolidate fix_auth#hardcode logic with recrypt logic


https://bugzilla.redhat.com/show_bug.cgi?id=1159009

/cc @jrafanie @Fryguy 
Why are we describing FixAuth::AuthConfigModel and then using subject below for FixAuth::FixConfiguration?  Is that just a typo in the extraction of the examples from the other spec?

If we're testing FixAuth::FixConfiguration, I'd prefer `describe FixAuth::FixConfiguration do` and `described_class.something` in your assertions.  I don't like subject in this case.