Description of problem:
The network labels on nic level are being added via the api:
/api/hosts/{host:id}/nics/{nic:id}/labels

When a setup networks command is sent via:
/api/hosts/{host:id}/nics/setupnetworks

The former labels are being removed from the nics.

How reproducible:
always

Steps to Reproduce:
1. Label a nic (via UI or REST)
2. POST a /api/hosts/{host:id}/nics/setupnetworks 

Actual results:
All the labels are being cleared from the host's nics.

Expected results:
The labels should be preserved

Additional info:
The setup networks api doesn't use the patch mechanism to preserve any interface property that wasn't sent by the user. Instead, it creates a new interface entity and set only the provided nic attributes.
There should be a 'labels only' patch to preserve the nic labels.