fix: object_handling method for overriding defaults

[alperenkose]

Description

This is an alternative approach for mitigating issues on passing None value to sdk for params which have default values in order to remove an xpath.

#587 PR and relevant pan-os-python changes would not be needed if we prefer this approach.

Motivation and Context

We are using default values for None provided params for newly created/replaced objects which can cause issue when SDK requires an actual None type input to remove an xpath.

For example pan-os-python does not accept 'none' string value for the nexthop type in StaticRoute but it requires nexthop type to be None type in order to be removed (in order words set to "None" in GUI), although it has a default value of 'ip-address'. Normally we expect to use default values from SDK when the param is None type but this kind of cases breaks this approach.

With the proposed object_handling() method, it is possible to override values after defaults are assigned so we can mitigate this kind of issues.

Also fixes #584

How Has This Been Tested?

Tested with manually written ansible playbooks.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.

[adambaumeister]

So to confirm my understanding, the issue here is that some objects in pan-os support being a literal "none" value, but right now we can't pass literal "None" from the modules as the default overrides it. This causes an issue in the circumstance when you want to set the type as "none" because we're always passing it a default. It it right?

[alperenkose]

The issue is pan-os-python expects a None type value being passed for some parameters when it should be removed from xpath. However in the current situation when a value is None type for non-existing objects the default value is being used for the param (e.g. when nexthop_type is None type it's converted to ip-address because of the default value for this parameter, however the intention was to remove/skip nexthop_type in the xpath)
This PR allows default values to be altered with an object_handling() method which can be overridden on specific modules if needed.

[adambaumeister]

lgtm