Tests: Fix failing test_ipalib/test_parameters
    
    Parameters test fails because of KeyError caused by improper manipulation with
    kwargs in Param.__init__ method. During initialization, if kwargs['required']
    or kwargs['multivalue'] is None, it is delete from dictionary and hence the
    missing key. Small change of the condition prevents this from happening.
    
    Partially fixes https://fedorahosted.org/freeipa/ticket/6292
    
    Backported to ipa-4-4 branch as per
    https://fedorahosted.org/freeipa/ticket/6395
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>