fixes modeling in zenoss 6.4.0 and higher

[romanbakaleyko]

fixes zps-6993

make modeling compatible with old and new adm code, preserve old style directives from PythonCollector for zenoss version < 6.4.0 to keep backward compatibility.

• fix the problem when there are two directives present in objectmap
• use directive _update == True instead of _add == False in case we want just to update existing component

[romanbakaleyko]

more details

First issue is that PythonCollector doesn't provide separate directive https://github.com/zenoss/ZenPacks.zenoss.PythonCollector/blob/f1597d4a96282485813ff86ed8e58ecafa158abe/README.html#L596 for update and create action. Based on the value of _add directive it means different thing action
_add == True means create component and _add == False means don't create component but update it if exists. New ADM code doesn't handle that, the problem I think is here https://github.com/zenoss/zenoss-prodbin/blob/develop/Products/DataCollector/ApplyDataMap/datamaputils.py#L63-L66 any directive with False value will be evaluated as nochange.

Second problem is in the same place, if we have two directives then ADM will use only first it finds.