Augments Protobufs for MidoNet models.

Adds missing Protobuf messages and fields for MidoNet models.
- Changed to Commons.IPSubnet to hold an address as IPAddress, not
  string.
- Added Subnet and Subnet Host to Network.
- Reflected Alex's Port message extensions.

Fix MN-2997
Change-Id: Ib09b488b44a87ea60211616193bc6bbe8c5b7d0a