CreateVM supports subnet specified

Currently the network info specified as part of server creation is limited
to network-id, port-id, and ip address.
When a network has multiple subnets then we need to also be able to specify a subnet-id.

Change-Id: I59cf7ee3c70694e14dfd42cdeba2cabddf2cab53
Implements: blueprint selecting-subnet-when-creating-vm
Merge "CreateVM supports subnet specified"