Need test coverage for VPN gateway

[lamchester]

VPN gateway ServiceManagmenent cmdlets are currently not covered by tests in azure-powershell, which means regressions in translating input parameters to C# Azure API calls will not be caught.

There have been no such issues so far, but test coverage would be good to have.

[markcowl]

@lamchester Please assign to a milestone

[markjbrown]

Chester, you can contact me and Deepak.