Added network argument to nova boot + fix id

Added support to nova boot for user to specify --nic net-id='public', --nic net-id='private', --nic net-id=private_vlan_id, or --nic net-id=private_vlan_id --nic net-id=public_vlan_id

where private_vlan_id and public_vlan_id are actual vlan ids.

Also converted id to string in subnet/network.