Add primary flag to 2015-06-15 NetworkInterfaceIPConfiguration

[seyadava]

Added primary flag to 2015-06-15 NetworkInterfaceIPConfiguration object on NetworkInterface.json spec.
This flags gets populated by api-version 2015-06-15 on Azure and AzureStack. So it is a bug that spec does not hydrate that flag.
Related issues:
Azure/azure-sdk-for-go#1997
#2578

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