Clean up the ```._resource``` field in the models

[kevinhillinger]

@dciborow submitted PR for azure/partnercenter-cli-extension#186 noting the issue with the syntax and that it violated linting rules.

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We need to continue to support out of the box behavior that the CLI gives us inheriting from msrest.serialization.Model while having a pythonic and clean way for it to ignore the field when serializing out from the CLI.

[dciborow]

Option 1: Rename _resource -> resource

class Offer(Model):
    _attribute_map = {
        'id': {'key': 'id', 'type': 'str'},
        'alias': {'key': 'alias', 'type': 'str'},
        'type': {'key': 'type', 'type': 'str'}
    }

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.id = kwargs.get('id', None)
        self.alias = kwargs.get('alias', None)
        self.type = kwargs.get('type', None)
        self._resource = kwargs.get('resource', None)

Usage: offer_resource_id = offer.resource.durable_id

But, then we should consider if the field should be adjusted the same way in 'Plan' and 'PlanListing'.

Option 2: Add method to retrieve durable_id

class Offer(Model):
    _attribute_map = {
        'id': {'key': 'id', 'type': 'str'},
        'alias': {'key': 'alias', 'type': 'str'},
        'type': {'key': 'type', 'type': 'str'}
    }

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.id = kwargs.get('id', None)
        self.alias = kwargs.get('alias', None)
        self.type = kwargs.get('type', None)
        self._resource = kwargs.get('resource', None)

    def durable_id(self):
        return self._resource.durable_id

Usage: offer_resource_id = offer.durable_id

This PR in my fork demos what this change would look like - https://github.com/dciborow/partnercenter-cli-extension/pull/15/files

(this is more pythonic then making a function def resource(self): return self._resource)