ipa passwd: use correct normalizer for user principals
    
    Commit c2af032c0333f7e210c54369159d1d9f5e3fec74 introduced a regression in the
    handling of user principals supplied to the`ipa passwd` command. This patch
    restores the original behavior which lowercases the username portion of the
    principal.
    
    https://fedorahosted.org/freeipa/ticket/6329
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>