IPA: default krb5_fast_principal to host/$client@$realm
If krb5_fast_principal is not set in sssd.conf it was set to host/$client,
KRB5 default realm was used which doesn't have to be the same as realm
used for IPA, thus authentication failed when using FAST.
Reviewed-by: Alexander Bokovoy <abokovoy@redhat.com>