Fix otp + force-join usecase

When the client already has a working keytab, use_otp is disabled. This creates
an issue when ipaclient_force_join is set, because the join module is called
with ipaadmin_principal and ipaadmin_password, but these variables may be
undefined if ipaadmin_keytab is used instead.
We should not disable OTP when force-join is specified.