This removes all references I could find to:
- credential provider
- authentication backend
- authentication provider
- auth provider
- auth backend
in favor of the unified:
- auth method
We've disabled this in the token store, but it makes no sense to have
that disabled but have it enabled elsewhere. It's the same issue across
all, so simply remove the ability altogether.
The /login path was confusing because its not relative and not consistent with other documentation. Other documentation (e.g., username and password at https://www.vaultproject.io/docs/auth/userpass.html) uses relative path.
