Update ServiceCreateSpec to use SubnetId
Update ServiceCreateSpec to use SubnetId

The service manager recently deprecated the use of vmNetworkId, and
instead will use subnetId. This switches the SDK to use subnetId. A
corresponding change will happen in the CLI.

Change-Id: If7425ce812ddd7945656567132a09f6164493ac4

vmware/photon-controller-go-sdk,vmware/photon-controller-go-sdk