Schema changes for VCPE feature.
-Define an object to represent subnet
-Define a link from this subnet object to virtual-machine-interface object

Change-Id: I2fdc56d4be68bec32f3b2f45ba735e9efea1f733
Merge "Schema changes for VCPE feature. -Define an object to represent subnet -Define a link from this subnet object to virtual-machine-interface object"