Now, if operators want to check some attribute isn't specified,
they should use 'eq' condition with None, '', etc values:
{'op': 'eq', 'field': 'inventory.bmc_addres', 'value': ''}
It would be useful to have condition plugin witch verify this case,
'is-empty' checks that field is empty string, list, dict or None value:
{'op': 'is-empty', 'field': 'inventory.bmc_addres'}
Change-Id: I6596a067e769530092c3db34405e0f0917d2f052
061d839a26