Add OTP as login option so user can use combo username+password+otp
to authenticate.
Use otp param at auth stage only.
Remove yubikey single-factor authentication.
Re-level password validation with otp check.
Add more info about otp as extras request params.
Merge pull request #10 from fedora-infra/feature/otp_login_method

Merging as discussed on irc last week.