[BUG] Virtual Network Gateway API changed

[ChenTanyi]

Unit test failed with VpnClientRootCertificateDataInvalid

Source:

• https://github.com/Azure/azure-libraries-for-java/blob/0818e69/azure/src/test/java/com/microsoft/azure/management/TestVirtualNetworkGateway.java#L302
• https://github.com/azure/azure-sdk-for-java/blob/0ba1e86ec13c17e9810757d93fb61b147fa68eb8/sdk/resourcemanager/azure-resourcemanager/src/test/java/com/azure/resourcemanager/TestVirtualNetworkGateway.java#L312