Bug 1451279

Cloned from upstream: https://pagure.io/freeipa/issue/6900

I'm getting an error when I try to add a yubikey to a user.  All of the commands below are run as root on the client.  Any suggestions on what I'm doing wrong or how to debug this?
```text
# ipa --version
VERSION: 4.4.4, API_VERSION: 2.215

# ipa -v otptoken-add-yubikey --owner=alice
ipa: INFO: trying https://test16.bbn.com/ipa/session/json
ipa: ERROR: non-public: KeyError: 'ipatokenotpdigits'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ipalib/backend.py", line 137, in execute
    result = self.Command[_name](*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 449, in __call__
    return self.__do_call(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 477, in __do_call
    ret = self.run(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 800, in run
    return self.forward(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipaclient/plugins/otptoken_yubikey.py", line 145, in forward
    cfg.mode_oath_hotp(key, kwargs['ipatokenotpdigits'])
KeyError: 'ipatokenotpdigits'
ipa: ERROR: an internal error has occurred

# ipa -v user-show alice
ipa: INFO: trying https://test16.bbn.com/ipa/json
ipa: INFO: Forwarding 'user_show/1' to json server 'https://test16.bbn.com/ipa/json'
  User login: alice
  First name: Alice
  Last name: Test
  Home directory: /home/alice
  Login shell: /bin/sh
  Principal name: alice
  Principal alias: alice
  Email address: alice
  UID: 431600007
  GID: 431600007
  User authentication types: otp, password
  Account disabled: False
  Password: True
  Member of groups: ipausers
  Kerberos keys available: True
```